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Thread: Lenses, zoom and focal length?

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    Default Lenses, zoom and focal length?

    Been window shopping for a DSLR. Wishing mostly. Have a technical question of sorts.

    My point&shoot has a 6x optical zoom. No idea what the focal length is. I just looked at a Nikon DSLR kit (waaaay over my budget) with a 200mm zoom lens that claims an 11x zoom.

    So can somebody explain the relationship between focal length and zoom, and maybe a little about the trade-offs between zoom and straight telephoto lenses?

    My main issue is there is frequently a railfan shot that I can't get physically close to. The 6x zoom on the P&S helps but I'd like to have more, and a better image anyway, but I know I won't be able to afford the really high soot telephoto stuff. So I'd like to understand the issues to make the best trade-offs.

    Not that I'll be able to pull the trigger anytime soon.
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    Well in the SLR DSLR world, a 50MM lens is "normal", so a 100 MM lens is 2X, a 200MM lens is 4X etc. But it gets complicated because the lower end DSLR's have a "cropped" sensor, so the lens magnification ratio is bigger. On a cropped sensor camera for the lens lenght you add one half of the lens length. So, on that entry level DSLR a 200MM lens is actually a 300MM lens, or 6X. A 50-200 zoom is really a 75-300 zoom, magnification wise. I have no idea where they get 11X from. As for the trade off's, a good zoom is fine unless your doing high end pro stuff and huge enlargements. If your not ready to buy I wont go any further

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    The zoom feature is used to change the focal length of the lense.

    As the focal length increases the images appear closer to the camera.

    As the focal length increases the depth of field also increases. The depth of field is the range at which the image is in focus. With a shorter focal length more of the foreground and background is out of focus. As an example if you use a 50 mm lense to make a portrait of your loved one, and you focus on the eyes it is possible that the ears and the tip of the nose will be out of focus. The best lense for portrait photos is about 80 mm focal length. A 35mm to 100 mm zoom lense with manual focus is a good choice for general photography and snap shooting. The reason I recommend a manual focus lense is because autofocus lenses do not always line up on the subject as you want it to.

    In the view screen of most cameras the entire image is always in focus. Therefore, the upper quality cameras have a Depth of Field Preview feature which you can use to see what is actually in focus on the recorded image.

    For long range telephoto shots a 100mm to 200mm zoom lense is good for hand held shots. Anything over 200 mm should be solidly mounted on a tripod to prevent blurred images due to microshaking of your hands and shutter bounce.

    Digital cameras also have what is called digital zoom. This does not bring the image closer to the camera. All it does is crop the image and enlarge it to the view screen. consequently the image quality suffers from low pixel count.

    There may be better cameras on the market, but Nikon has been the industry standard that all other cameras are compared to. The best testimonial to that is the fact that National Geographic Magazine's photographers all use Nikons. There are some who will recommend Canon cameras for good reasons. They make great cameras also.

    If you have more questions....ask them.
    (The voices I hear in my head may not be real, but sometimes they come up with a good idea.)

    Have fun.

    Moose

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    Quote Originally Posted by MooseID View Post
    The zoom feature is used to change the focal length of the lense.

    As the focal length increases the depth of field also increases. .
    Actually its the opposite, the most DOF is attained at short focal lengths.

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    Correction. As focal length increases, depth of focus decreases. Wide angle lenses (less than 50mm focal length) have very wide depth of focus. They sometimes don't have to be focused and practically everything is in focus. With telephotos you have probably seen photos of wildlife where the animal is sharp and everything else is out of focus, making the subject standout. This is due to the very shallow depth of focus. Portrait lenses are best in the 85 to 100mm range because they provide adequate depth of focus without distortion of the image. When used to photograph people, they are used at sufficient distance to provide depth of focus that covers the whole head front to back without distorting the nose, such as a wider angle lens would do.

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    Good answers so far. Thank you.

    On one specific point. My pont & shoot has a 6x optical (not digital - I never use digital) zoom. What you're saying is that to get an equivalent "bring that boxcar closer to me" from a DSLR lens I need at least a 300mm focal length. BUT on the lower end DSLR because of the way the sensor is cropped I might get away with a 200mm lens.

    Is that right?


    How do i know if the sensor is cropped on a particular model?
    Never mistake a guy who talks a lot for a guy who has something to say...

    CH&FR Site and Blog: http://www.chfrrailroad.net and http://blog.chfrrailroad.net
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    I think kdk is correct that a 200mm lens on a Nikon is around 6x. If you're looking mainly at optical magnification, the most affordable option is probably a so called super-zoom point-and-shoot camera (see this article in PC Mag for some examples). Otherwise if you had a DSLR already I would suggest looking into renting a telephoto lens, as they are too expensive to purchase for casual use.

    That said, the Nikon 55-200 with image stabilization is said to be a good deal for the money. With the bigger sensor on the DSLR, you might be able to crop tightly on your computer enough to mimic a longer telephoto and still have a good quality image.

    How do i know if the sensor is cropped on a particular model?
    It's kind of the opposite. All Digital cameras are "cropped" compared to film cameras unless it specifically says "full frame" and usually you only find those on the Pro level DLSRs.
    Last edited by jmodule; 29th Dec 2010 at 05:51 PM. Reason: replied to more recent post
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    If we are going to get into the arena of DSLR "wannabes" then I have to suggest my Fujifilm SD2500HD https://www.nscale.net/forums/showthr...S2500HD-Camera

    I couldn't be happier with it and a very reasonable price!

    Edit: BTW, that PC Mag article is missing some very viable contenders! (Maybe it is dated?)

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    There are a number of things that effect depth of field. Lens focal length is one and has already been discussed. The other major thing that effects depth of field is aperture, or size of lens openning that light passes through. It is also sometimes called f-stop. If you see it listed it often times has an "f" in front of it, f2.8 or f22. In general the larger the lens openning (the lower the f number) the less depth of field, the smaller the lens openning (the greater the f number) the greater the depth of field. It is not unusual for a zoom lens to change the maximum available lens openning as focal length changes.

    How do i know if the sensor is cropped on a particular model?
    Normaly the camera specifications will tell you sensor size. Full frame usually indicates a sensor that aproximates a 35mm film size of 35x24mm. APS-H (used in some canons I think) is 28.7x19mm, APS-C is probably the most common of the smaller sensors and varys in size 23.6x15.7mm and 22.2x14.8 are popular. Thanks to the Wikipedia for the sensor size information.

    Dave

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    How do i know if the sensor is cropped on a particular model?
    The Nikon DSLR's are sensor cropped (DX)with the exception of these models: D700 and the D3xxx series. These models are
    full frame (FX) like the older 35mm SLR cameras. A 50mm lens on a D700 or D3xxx will actually be 50mm while on all other
    Nikon DSLR models it would be 75mm with the sensor cropping (DX). The full frame models are usually quite a bit more in price than the
    DX models.

    Note: The D300 and D300s are not in the D3xxx group. They are both cropped (DX). The D3xxx grouping is the D3, D3S, and the D3X. I think
    I got them all.
    Last edited by Dennis R; 29th Dec 2010 at 10:34 PM. Reason: clarification
    Dennis

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    TD,

    When camera makers say "6x zoom" they mean the longest focal length of the lens is 6x the shortest focal length. You didn't give the shortest focal length of that 200mm zoom. If the longest focal length is 200mm and it is truly an 11x zoom then the shortest focal length would be 18mm. (Just divide 200mm by 11.)

    As stated in a previous post most DSLRs have sensors smaller than a 35mm film frame, typically resulting in a 1.6 magnification factor. (It really isn't magnification, just a change in the field of view.) So if that Nikon lens is an 18-200mm zoom and the DSLR you were looking at has a 1.6 factor then the lens would give the same basic view as a 29-320mm zoom on a 35mm film camera (or a DSLR with a "full-frame" sensor). That is a pretty nice zoom range that would cover most photographic situations.

    --Sherman

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    Quote Originally Posted by TwinDad View Post
    Good answers so far. Thank you.

    On one specific point. My pont & shoot has a 6x optical (not digital - I never use digital) zoom. What you're saying is that to get an equivalent "bring that boxcar closer to me" from a DSLR lens I need at least a 300mm focal length. BUT on the lower end DSLR because of the way the sensor is cropped I might get away with a 200mm lens.

    Is that right?

    Thats correct Twindad


    How do i know if the sensor is cropped on a particular model?
    You would need to do a little research Twindad but, Nikon calls it a "DX" sensor, the cropped sensor, verses an "FX" sensor, but basically, if you dont spend over 2000.00 for just the body your going to get a cropped sensor.

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    Keep in mind - I'm not shopping, and won't be any time soon. All of this stuff is WAY out of my budget. But I'm dreaming, and trying to understand.

    So the Nikon I semi-randomly chose was a package deal of a D700 (12.1MP FX format sensor) and a Nikon AF-S DX Nikkor 18-200mm f3.5-f5.6 zoom lens (good guess, Sherman).

    The camera in my hand is a Canon PowerShot A710 (7.1MP sensor) with a "6x zoom lens" - here's where it gets confusing. the front of the lens case says: "5.8-34.8mm 1:2.8 - 4.8" ... but a check at www.dpreview.com says that its effective zoom range is 35-210mm f2.8-4.8. I'm guessing the review site is giving the effective focal distance range after the DX-ish cropping of the sensor?

    The same review site gives a "35mm equivalent focal length (APS-C)" for the Nikon LENS of 27-300mm, a maximum aperture of f3.5-5.6 and a minimum aperture of f22-36.

    OK, I have no idea what APS-C means... but it also says the maximum format size is "DX"... I expect this means that if you plug this guy into an FX format camera body like the D700, the LENS is going to crop the image for you (or force the camera to do it or whatever).

    So looking strictly at my "bring that boxcar closer" metric... let's say the boxcar appears to be 1000 feet away through the Nikon lens at the 27mm wide angle view. If we say the Nikon lens's 27mm wide angle as "1:1", then...

    • The Canon wide angle shot would be at about 1:1.3 (35mm/27mm) - the boxcar will appear to be about 770 feet away (close to its actual distance, coincidentally)
    • The Canon zoom shot would be at about 1:7.77 (210mm / 27mm) - the boxcar will appear to be about 129 feet away
    • The Nikon at full zoom would appear at 1:11 (300/27) - the boxcar will appear to be about 90 feet away.

    Now, to put this in real terms... in order to get that same "90 feet away" shot from my Canon, I have to get... 300 ACTUAL FEET CLOSER to the subject, yes? (90 * 7.77 ~= 700, while 90 * 11 ~= 1,000). Or flipped around I can get the same shot in the Nikon from 300 feet farther away...


    Right?

    Nevermind how much better it would look due to the better optics, sensor, and so on with the higher-zoot camera set.

    And I'm guessing based on remembering using dad's camera, that much over about 300mm the lenses get in the "rent it" range due to size and cost... (the Canon L-series 500mm telephoto weighs 8.5 lbs and costs $7,000 at Best Buy... I don't think so...)

    And yes, BryanC, I will look at the Fuji. Though I'm a bit of a Canon fan - my dad has had them since I was a kid.
    Never mistake a guy who talks a lot for a guy who has something to say...

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    The same review site gives a "35mm equivalent focal length (APS-C)" for the Nikon LENS of 27-300mm, a maximum aperture of f3.5-5.6 and a minimum aperture of f22-36.

    OK, I have no idea what APS-C means... but it also says the maximum format size is "DX"... I expect this means that if you plug this guy into an FX format camera body like the D700, the LENS is going to crop the image for you (or force the camera to do it or whatever).
    TD, the APS-C refers to the sensor format of 25.1 X 16.1mm which is DX on a Nikon. The 18-200 will not be cropped on a D700 so you will not get the 300/27. Also, a DX lens on a D700 will decrease the photo down to 5MP instead of the normal 12.1MP size. The photo would still have good quality however. Vignetting (black edges) will also occur at certain focal lengths.
    Dennis

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    Quote Originally Posted by Dennis R View Post
    TD, the APS-C refers to the sensor format of 25.1 X 16.1mm which is DX on a Nikon. The 18-200 will not be cropped on a D700 so you will not get the 300/27. Also, a DX lens on a D700 will decrease the photo down to 5MP instead of the normal 12.1MP size. The photo would still have good quality however. Vignetting (black edges) will also occur at certain focal lengths.
    Interesting... so if I put the 18-200 lens on a D700, I get 18-200 and half the MP, but if I put it on a DX body, I get 27-300 and all the MP. Presumably a good argument for using the (higher quality/price?) FX capable lenses on the D700, or a (cheaper?) DX body on this lens?

    My head is swimming...
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    TD, not all the FX lens are more expensive. There are some very high quality FX lens that are cheaper than some of the DX lens. The 50 1.8 ($120), 70-300 VR (500-600), the 60mm macro (550), plus others are all excellent FX lens and will work on both FX and DX. They would all still be cropped on DX body. A 17-55 DX lens sells for about $1400.00, a very high quality DX formatted lens.
    Dennis

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    So... umm... can y'all explain this FX vs. DX thing a little more? How exactly is the image cropped? Is it done with some kind of mechanical aperture, or in software, or by shutting down part of the CCD? What are the side effects and trade offs? What's going on here?
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    TD,

    The smaller sensor is the DX sensor, the "full-frame" sensor is the FX sensor. Full-frame refers to the sensor being the same size as a frame of 35mm film.

    The lens situation works like this- a lens projects an image circle on the film or digital sensor. Since the DX sensor is smaller the lens only needs to cover that sensor size, so a DX lens would have an image circle just slightly larger than the diagonal dimension of the sensor (slightly larger to avoid light fall-off in the corners that would cause vignetting). An FX lens needs to cover the full-frame sensor so it has to have a larger projected image circle. All things being equal (and they rarely are) a lens with a larger image circle is more expensive. It is more difficult to design say a 100mm lens with an image circle of 40mm than one with an image circle of 29mm.

    Now take it the next step- if you put a DX lens on an FX camera the lens will not illuminate the entire sensor (since the DX lens has a smaller image circle than the FX sensor's diagonal measurement). So you get a "cropped" image; actually an image that is vignetted on either end.

    My digital system is Canon and they have the same setup, EF lenses project an image circle large enough to cover a full-frame sensor, EF-S lenses project a smaller image circle. With Canon cameras you can mount an EF lens on either a full-frame camera or a smaller sensor camera since the EF lens will cover both sensors. However you cannot mount an EF-S lens on an EF sensor-sized camera since it won't cover the larger format. I'm surprised that Nikon lenses will mount either way.

    And yes, when you look at the review of your camera and it says the effective zoom range is 35-210mm it is expressing the zoom range as a 35mm or full-frame equivalent. It is the best way to compare the focal lengths of cameras that have vastly different sensor sizes.

    --Sherman

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    Apparently you can't double-thank a post. If I could, I would. Lightbulbs coming on all over the place now.

    So... What aboutthe focal length/zoom question? Am I right that I would need to get 300 feet closer to my boxcar with the 200mm vs the 300mm?
    Never mistake a guy who talks a lot for a guy who has something to say...

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    TD,

    How much closer you need to get depends on how far away you started in the first place. Keep in mind that the focal length number 200mm, 300mm etc. doesn't have anything at all to do with the distance to the subject. The things to consider are "angle of view" and "field of view". A 200mm lens on a 35mm camera has an angle of view of about 10.1 degrees. A 300mm lens has an angle of view of about 6.5 degrees.

    At 300 feet from your subject a 200mm lens on a camera with a 1.6 multiplier has a field of view of 33.80 feet. On the same camera at the same distance a 300mm lens has a field of view of 22.5 feet. To get the same image with the 200mm that you got with the 300mm you would need to be 200 feet away.

    Here's a site with some useful photo calculators.
    http://tawbaware.com/maxlyons/calc.htm

    --Sherman

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