DX vs FX how to calcuate the focal length?

[laurence_nyein]

<p>Hi,,,<br>

I know there are many image sersor format eg. DX format , FX format which i believe is 35mm or there are larger image sensor. Here, i just want to concentrate on Nikon's DX and FX format. I was trying to find out how the focal length differs by using different types of cameras with a lens. I will make it simple. For example, Nikkor 50mm AF f1.4 lens, if i use that lens on DX format camera, the focal length 50 mm become 75 mm. (or is equalvalent to) Is that correct? so if i use 105 mm lens (which is not DX lens) to a DX format camera (eg. D90, D300, etc ) what is the focal length i can get will be? How to calculate those ratio? or can i have that in a table comparing all the different focal length for DX and FX? thanks. PS. i tried to find that on google but now i got headache so i finally try to post the question :P. thanks again.</p>

[dpbours]

<p>The factor 1.5x for DX cameras is for any lens - whether it is a specific FX digital lens, DX digital crop lens or old full frame film lens. FX and old full frame film lenses (Ai AiS AF-D G-grade etc etc...) are the same, but with over 20 years development in between, the new lenses often have better coatings and nifty electronics like VR. Sometimes old glass is as good or even better though...<br></p><p><br></p><p>The focal length on the lens is always written with respect to the 35mm equivalent, ie. full frame. </p><p>That is done for DX specific lenses, as it is done for the four-third lenses or micro four-third lens cameras. As such, your 105mm full frame lens would be 157mm on your DX camera.</p>

[douglas lee]

<p>Sensor size, DX or FX, does not affect focal length. A 50mm lens is a 50mm lens regardless of camera format. What you are referring to is the field of view (FOV). A 75mm lens on a Nikon FX camera body will have the same FOV as a 50mm lens on a Nikon DX body. The ratio is 1.5. So, a 100mm lens on a DX body would have the same FOV as a 150mm lens on an FX body. Hope that helps.</p>

[dpbours]

<p>A good link on all the Nikon abbreviations: http://en.wikipedia.org/wiki/Nikon_F-mount</p>

<p>And a nice one on crop factors and sensor sizes: http://en.wikipedia.org/wiki/Sensor_sizes</p>

<p>But you are right, no one tells you that the focal length on the lens is still written in 35mm equivalent...</p>

[dpbours]

<p>And yes, Douglas is completely right; Focal length and field of view are the correct terms!</p>

[laurence_nyein]

<p>thanks Dennis for your explanation. That really helps.<br>

Thanks douglas for your correction for my mis-usage.<br>

Now it's all clear. As u can see i m a beginner to photography and own a D90 and now i m collecting lens but i want to collect lens that can use on FX camera like D700 or D3 coz i may be upgrade to one of those cameras later time, so i can keep all my lenses.<br>

Thanks again guys.</p>

[dpbours]

<p>Very smart to go for non-DX! I started with a D200 and an 18-200 DX lens, which was worthless when I went to the D700... An FX on a DX camera has other advantages: If the lens is soft on the edges on a full frame camera, this is not the case on a DX camera, since the edges are not used...</p>

<p>But there is very good glass in old Nikon lenses. I got the old 50mm 1.4 (200 euros second hand). Wonderful. I'm addicted to that lens. Then the 105mm DC 2.0. Defocus control lens, only Nikon builds DC lenses. Very nice, though you need to be lucky with a good price second hand. Got mine for 400 euros.<br>

I also got an 85mm 1.8 and 20mm 2.8, since prices are very reasonable second hand for these lenses. Then the Nikon 18 - 35 is f/3.5 - 4.5 (250 euros second hand), but is quite sharp and a very light lens. Great for travel.</p>

<p>For the other zooms, I got second hand Sigmas. The 24 - 70 (300,-) and the 70 - 200 2.8 (450,-). Great glass for the money.</p>

<p>The only two new lenses I bought are the 14-24 (1200,-) and 80-400 (1400,-) of Nikon.<br>

Both lenses for specific purposes.</p>

[ShunCheung]

<p>The problem with buying all lenses that will fix FX for your current DX camera is that the focal lengths might not work all that well as long as you use DX cameras, especially for zooms and wide angles. For example, you may have to go all the way to the 14-24mm/f2.8 AF-S to get a good super wide for DX, while there are many more affordable DX super wide zooms.</p>

<p>In particular, a few years down the road, it is likely that a lot of FX bodies will have 20+MP or perhaps even more. Those cameras are going to be very demanding on lens quality or you'll be wasting the pixels. Be careful if you stock up on affordable FX lenses today. You may have to replace them all anyway down the road.</p>

[dpbours]

<p>You're absolutely right about that one!<br>

But a razor sharp f/2.0 lens of 1985 will still be a very decent lens on a 20+MP sensor. Off course, you shouldn't try any of the less light sensitive models or lenses that already had CA's and edge sharpness issues back then...</p>

[Peter_in_PA]

<p>BUT...</p>

<p>You really ought to have a "standard zoom" for most general photography, and on a DX camera, that will almost certainly be a DX lens. Don't be afraid of buying one. You have NO idea how long it will be before you upgrade to FX, if you do at all. Many of us don't need to and don't plan to. For instance, the "useless" 18-200" mentioned above could be sold used and at this point you wouldn't lose any more significant amount of money than if you sold an FX zoom of some kind.</p>

<p>Don't be in such a state to acquire only FX lenses that you don't even outfit yourself with what you need to take good photos now. Certainly, I think you should think twice about acquiring something like a 24-70 f2.8 that you "might" use on FX. It's not a great general photo range on DX. With a D90, an 18-70 for instance, for a lot less money, will give you a more generally useful range.</p>

[ShunCheung]

<blockquote>

<p>But a razor sharp f/2.0 lens of 1985 will still be a very decent lens on a 20+MP sensor. Off course, you shouldn't try any of the less light sensitive models or lenses that already had CA's and edge sharpness issues back then...</p>

</blockquote>

<p>Not necessarily. For example, my test of the much praised 14-24mm/f2.8 AF-S on the 24MP D3X shows that lens is not that great near the corners, a problem not shown on the 12MP D3 and D700. My observation confirms the result from other well known testers.</p>

<p>Of course, today, the price for the D3X is sky high so that few can afford one to notice these issues. However, in a few years when you can buy a 20MP or 30MP FX DSLR for $2000, such lens limitations will be better known. Consider that a 21MP Canon 5D Mark II is already in the $2000+ range for a year, we are not all that far from $2000 high-pixel-densithy Nikon DSLRs.</p>

[douglas lee]

<p>Keep in mind that you can use DX lenses on a FX body. The Nikon bodies will automatically switch to DX mode. You won't be using the entire sensor area, however. Or, you can keep the camera set to FX and see what happens. The new Nikkor 35mm f1.8 DX works very well on the D700. There is some some edge darkening, but I like it on some images and it can be corrected in post. Just a thought,</p>

[Peter_in_PA]

<p>Douglas, sorry... The 35mm AF-S does <strong><em>not</em></strong> have an acceptable image circle for any serious FX use. If you have very low standards for edges and don't mind an uneven vignette that gets totally unusable by f8, then maybe, but you will get better "vignetting" results if you add THAT in post to a 35mm F2 lens or some such. Planning to use that lens on FX is a mistake. And the serious vignetting in an image like the one I reference below can NOT be corrected in post at all.</p>

<p>Read this and look at Shun's examples. http://www.photo.net/nikon-camera-forum/00VLHs</p>

[steve m smith]

<blockquote>

<p>The focal length on the lens is always written with respect to the 35mm equivalent, ie. full frame.</p>

</blockquote>

<p>The focal length written on the lens is always the actual focal length of the lens - the format doesn't matter. The apparent focal length expressed in 35mm (FX) terms is a different matter.</p>

<p> </p>

[douglas lee]

<p>Peter, for serious FX use, I agree with you. However, there are situations where the lens is usable on FX. I believe Arthur pointed them out in the thread you referenced. You disagree, fine. I have no problem with that. Take care, Doug</p>

[ShunCheung]

<p>I'll repost what Bjorn Rorslett wrote in the previous thread about using the 35mm/f1.8 AF-S DX on the full FX frame:</p>

<blockquote>

<p>A person can do whatever he or she wants in their own shooting. Make a superlong lens hood to maximise vignetting is OK if that's what you want.<br>

However, claiming the following "'On the FX body, it works perfectly in FF mode with some vignetting wide open#" is not only confusing, it is strongly misleading for the 35/1.8 DX. The vignetting and quality loss are severe. Not surprising since this is a DX lens. People may put it on an FX body to get lots of vignetting - fine - but don't pretend the lens is suitable for straightforward FX work.</p>

</blockquote>

<p>If one would like to get the serious vignetting effect and does not care about edge performance, it is your choice. There are people who add vignetting in post processing because that is the effect they want. There are also people who use "Lens Baby" to get special effects of a twisted, unsharp image; it is all fine. However, when one suggests to use a DX lens on FX, including the 35mm/f1.8 DX, it is important to point out the effect and limitations. Otherwise, it is highly misleading to leave the impression that such combination is acceptable for general FX work.</p>

[douglas lee]

<p>I did not mean to mislead anyone. As I re-read my earlier post, I see that I may have done that. My apologies.</p>

[laurence_nyein]

<p>thanks guys for your suggestions and explanations. that's fantastic.<br>

What happened and what led to this question is I got a D90 with package lens which is 18 to 105 mm zoom dx. And i bought 105 micro (not DX) but i didn't notice the difference between DX and FX. And i bought 50 mm fixed f1.8 D lens (not DX) as i saw it on a website and the price is quite cheap so i went for it. Ohh. before i bought it i tried to find out the distance with my package lens. I put the focal lenght to 50 and tried to shoot few photos in my home studio for some portriate shoots. That's all fine. So i bought the lens (50 mm lens). And i found out images are abit closer then when i tried with my package lens at 50 mm zoom. As my home studio is just built in a small room of my house so i can't get back enough to fit the person enough. So that was when i started asked this question.<br>

One more mistake i made about this lens is, it said f1.8 but the images are too sharp for a f1.8 lens. I should have bought f1.4 50 mm for abit more money. Well, the price is double here between that two lenses.<br>

Thanks guys , your explanations and comparisons between Fx and Dx cameras and things are really great. c ya around.</p>