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Thread: Is a 35mm DX lens 35mm on a DX camera?

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    Member coolhand78's Avatar
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    Is a 35mm DX lens 35mm on a DX camera?

    Or is it 52.5mm if the crop is 1.5...?

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    Moderately Underexposed I @ M's Avatar
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    The question has been asked many times ----

    You cannot change the physics of the situation.
    The lens is a 35mm focal length lens whether it is mounted on an FX or a DX body.
    What does change however is the field of view that the lens projects on to the sensor.

    There are two very worthwhile pages in the library that explain the situation very clearly.

    http://www.ausphotography.net.au/for...ctor_w_example

    http://www.ausphotography.net.au/for...vs_Crop_Factor
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    As Andrew says, there is not some magic and trickery behind it. Any lens is the same no matter what camera body it is mounted to. It is just the sensor that may be different. As per Andrew's links it is about the field of view. So a 35mm lens is a 35mm lens, no magic simsalabimb stuff makes it a 52mm lens automagically.
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    Arch-Σigmoid Ausphotography Regular ameerat42's Avatar
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    Cools. Another word you'll hear i discussions like this is "reach". Take it to mean to reach for an aspirin, or something like that.
    Although, a lens of a given FL may "act" like one of a longer FL on a crop body, it's only the Field Of View (FOV) that's the main part of the similarity. Think more of the resulting image size on the sensor rather than the relative image size in the frame.

    Where the comparison is useful - well, maybe to ex-film camera users - is what FL on a crop body would be good enough for portrait shots. In these, you are trying not to get too wide and close a view that it distorts facial features by way of "differential magnification".

    Well, some comparisons are valid, and some are not.
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    Thanks guys, so basically If I wanted a lens that yielded the same FOV as a 50mm lens on an FX body I'd really need something around a 35mm FL on a DX..?

    Or another way to see it is that an FX sensor/camera will see 1.5x more than that of a DX camera with the same lens, but all the other characteristics of that lens will remain the same, DOF, FOV, distortion etc etc?

    And having said that will using a 35mm lens on a crop sensor have different characteristics, more or less, to that of a 50mm lens on a full frame/FX camera?

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    A royal pain in the bum! arthurking83's Avatar
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    Just one reason that 'some of us' like to stick to traditional understandings of lenses and what they are, I'll give you an example of why a 35mm lens on Dx is not 'equivalent' to a 50mm lens on full frame.

    Firstly, while it's true that a 35mm lens on APS-C (sort of) looks like a 50mm lens on full frame due to the field of view, it doesn't act like one.

    Consider the example where you have a full frame camera, and prefer to use a 35mm lens, it will produce a specific DOF, and outlook 'perspective' due to it's focal length.
    But if you crop this image(from the full frame camera) to an equivalent scale to mimic an image of a 50mm lens on full frame ... does that instantly make this image one that is shot by a 50mm lens?

    Of course it doesn't, the lens used is 35mm .. and basically that's what a 35mm lens on APS-C is doing.

    So if a 35mm lens is used on full frame and then cropped to an equivalent FOV of a 50mm lens, we don't instantaneously describe that image as shot with a 50mm lens.

    And what if you cropped this full frame image to some random percentage, and it then has an equivalent 43mm FOV instead .. as you can see from this sort of example, the infinite possibilities of random post processing(in the form of cropping) would make for a nightmare situation trying to keep a track of what focal length is/was used .. and so on.

    To an APS-C user, the fact that the 35mm lens achieves a FOV that looks roughly similar to a 50mm lens on full frame is of no consequence in real world usage .. as they probably don't know what a 50mm lens FOV is on full frame.

    Where this commonly proposed usage comes into play, is for longer focal length uses, where a 200mm lens on a cropped body gave the illusion that you had more reach.
    And because cropped sensors seem to have higher pixel densities people were under the illusion that they had more magnification.
    This has/had nothing to do with the lenses focal length, it was (always is) an issue of the camera.

    As an easy example of what this means, and how it doesn't actually work in real life consider the D800 and the D7000, the D800 has 36mp, and the D7000 has only 15Mp.
    But in Dx crop mode, the D800 also has approximately 15Mp .. so a 200mm lens on a D800 has the same reach as it does on the D7000 .. it has no more and no less magnification on either camera body.

    So there are good reasons to not fall into the trap of this misconception that the camera format has any bearing on the lenses optical properties.
    But keep in mind that when you choose the camera format you want to use, you'll get different views when using a specific lens.

    I commented in another thread on this topic that we(or someone) should post a thread to dispel the commonly held misconceptions about this topic. If I get the time too, one day I will, but the time is hard to come by.

    Don't purchase a lens based on what it's apparent FOV is. This isn't the best manner for deciding what lens is best for your usage. Get a lens based on whether you need it to be wider or closer or that it renders in a specific manner, or that it has a certain aperture range, or that it can focus closer, or render more sharply.
    But to factor in the situation that a lens of XXmm is what's needed because it gives a FOV equivalent to YYmm lens on another format is not good practice!
    if you need a 50mm lens, then get a 50mm lens. If your instructor says you need to shoot with a 50mm lens, she doesn't mean that you should take into consideration that you have a cropped sensor and must therefore use a shorter focal length lens to compensate.
    If you choose to compensate for the cropped body, and require it for any sort of technical application and this improper usage of lens focal length is not taken into consideration, the technical aspects of the image will be out of whack with the expected results.
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    Moderately Underexposed I @ M's Avatar
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    You MUST take into account which format the lens is built for!

    An AF-S DX NIKKOR 35mm f/1.8G lens will give some vignetting on an FX body due to the image circle being slightly smaller than a AF-S NIKKOR 35mm f/1.4G
    as the F/1.8 lens is built specifically for DX bodies and it is designed to cover the APSC (DX) sensor.
    The F/1.4 lens is made for FX bodies and will work perfectly on both DX and FX bodies.
    Both lenses will provide around the 50mm field of view on DX bodies.

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    A royal pain in the bum! arthurking83's Avatar
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    Quote Originally Posted by I @ M View Post
    You MUST take into account which format the lens is built for!

    An AF-S DX NIKKOR 35mm f/1.8G lens will give some vignetting on an FX body due to the image circle being slightly smaller .......
    Just had a further brainwave on the matter Andrew.

    To illustrate how ridiculous the notion that a 35mm lens on APS-C is equivalent to a 50mm lens on full frame ... save for the fact of the cropped camera limiting the FOV of the lens ....

    The AF-S DX Nikkor 35mm f/1.8G lens can almost form an image on a full frame camera, save for some vignetting.
    If I shot using this lens on a full frame camera, set up a batch process to eliminate the vignetting .. does that give me a really cheap AF-S Nikkor 37mm f/1.8G lens?
    ( after all, I'm only cropping out a 2mm FOV equivalency! )



    .... actually, I'm in the market for an interesting new lens!

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    Moderately Underexposed I @ M's Avatar
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    Quote Originally Posted by arthurking83 View Post
    .. does that give me a really cheap AF-S Nikkor 37mm f/1.8G lens?
    ( after all, I'm only cropping out a 2mm FOV equivalency! )
    Yes.

    The little 35 F/1.8 does a very good job on an FX body but only if you leave the lens hood off.

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    Thanks very much for the responses, it's definitely making more sense to me now, whilst at the same time making me more unsure whether I should get the 35mm f/1.8G or the 50mm... I'd settled on 35 as I thought I'd be getting something like a 50mm on a full frame which was what i thought I needed... For street and landscape stuff...

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    A royal pain in the bum! arthurking83's Avatar
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    Quote Originally Posted by coolhand78 View Post
    ...... whilst at the same time making me more unsure whether I should get the 35mm f/1.8G or the 50mm... I'd settled on 35 as I thought I'd be getting something like a 50mm on a full frame which was what i thought I needed... For street and landscape stuff...

    My response was partially aimed at that line of thought.

    The question is, why do you think you need a 50mm lens. Everyone has differing tastes, and I have 3 50mm lenses, all of which hardly ever get used .. and one of them is my fastest lens!. Not to mention many varied zoom lenses that do 50mm as well .... I used to be predominantly a landscaper, looking for the widest lens I could get, and it didn't bother me if it were 14mm, 10 mm or 8mm ... just wanted it wide.

    I'm now more into medium telephoto focal lengths, and trying to use them more .. but again, it's not a specific focal length fixation .. I have various new lenses from 75mm up to 135mm, and they each do about the same thing ... just a little differently to each other.

    35/1.8 is at least cheap ... in my opinion a nice lens but somewhat limiting. 28mm on Dx is a bit more usable in many situations, 24mm felt about the best compromise between being wide, but not overly so with dramatic distortion ... and the ability to shoot portraits. Everything in between, from a bit of landscaping to (smallish)group shots. 24mm on Dx is closer to a 35mm FOV on fullframe, and 28mm(~42mm). But as I already said, if you haven't got a 50mm lens on a full frame camera to compare and directly visualize what that means ... the point is moot.

    Go to a store, try a 35mm; try a 28mm ... try a 24mm lens too ... in fact try them all and see for yourself which one works best for 'ya.
    A 24mm f/1.4 would be my ideal one of fast prime lens for Dx(I reckon) .. and all others would almost certainly be zooms ... well maybe one more longer portait-ish lens that doubles as a macro.

    But if you can hold off for a while longer and wait for the Sigma 18-35mm f/1.8 the test/review so far produced(albeit on a pre production lens!!) is quite astonishing.

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    I'm interested in street photography, and i'll be on holidays in a couple of weeks (honeymoon) trekking around Hobart and Launceston and then i'll be back in England next
    year and travelling around Europe a fair bit, and i've been reading and watching a lot of reviews/how to's etc and the 50mm is the one that people seem to like for that type of thing.

    I will have the 16-85mm f/3.5-5.6 as well which will probably the main lens but sometimes i think i'd like to just use the prime when wandering around. The 35mm and 50mm nikkor f/1.8's are both cheap lenses but i can only afford one of them, the 24mm is completely out of the price range unfortunately, as that does look like a fantastic lens, maybe down the track a bit

    so i'm back to the old question now - 50mm or 35mm, i do want a prime lens as it's something that i think will appeal to me and its something that i would like to explore, thankfully as mentioned they are cheap and reasonable from what i've read, but I'm just confused as to which one will be best suited for street/architecture type shots on a D7100 DX camera...???

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