Small Sensor Crop Factor (DX Lens Crop Factor)

Discussion in 'Nikon' started by john_lai|3, Jun 19, 2008.

  1. Sorry guys if this topic has been discussed before but I am still a bit confused about the DX's 1.5x crop factor. A 18-200mm VR DX lens means 18-200mm right? A 70-300mm VR Lens (not DX) means 70*1.5 - 300*1.5 mm right? Lets say if there is a 18-200mm VR lens and a 18-200mm VR DX lens, I would be able to get 18*1.5 - 200*1.5x mm on the non-DX lens? Thanks in advance. John
     
  2. Nope, you apply the crop factor to ALL lenses... even DX branded ones.
     
  3. SCL

    SCL

    Don't think of the lens as doing the cropping...that is the sensor. The DX lenses merely are designed to spread the light onto the smaller sensor rather than a larger one (simplistic explanation ignoring other characteristics).
     
  4. A lens that is 24mm on a film camera still is a 24mm lens on a digital camera with a 1.5 factor. The difference is that the digital camera crops the FIELD OF VIEW to what a 36mm lens would see. You are only using the the center 2/3 of the lens.
     
  5. Isn't the "crop factor" [equivalent field of view] 1.6x, not 1.5x, on an APS-C sensor? It's 1.5 on a 4/3-system sensor.
     
  6. Sorry, senior moment! It's 2x on a 4/3 sensor.
     
  7. crop 1.6x is with the other brand.....
     
  8. Nikon labels SLR/DSLR lenses by their actual focal lengths. They do not use "35mm equivalent" focal lengths, like some P&S cameras are labelled. They might have, but they don't. So "DX" is irrelevant in figuring out the field of view of a lens on a DX camera.
     
  9. The "crop factor" is with respect to a 35mm film or full sized sensor. As was noted, it has nothing to do with the lens, really, but how much of the scene hits the sensor and how much is too wide or too tall and misses the sensor. This changes the field of view of the final picture. We say it has the equivalent view of a lens 1.5 times longer. So, a 50mm would end up with a field of view of a 75mm lens. Now, the issue comes when you put a DX lens, which is designed to only send light the same size as the digital sensor, on a full sized sensor or a film body. In this case it doesn't fill the whole space up, and you see that on the result.
     
  10. I'm gonna throw my hat in this too :) If you put a 18mm FF lens on a DX body it will look the same as if you put the 18-200mm on the DX body and set the lens to 18mm. But if you put that 18mm on a film camera, it would be much wider. Wider FOV.
     
  11. Thanks for the answers.
     
  12. I added this into a post a few days ago to show the difference between DX and FX sensors and lenses.
    00PtFs-50541684.jpg
     
  13. Michael I think I finally understand now. It seems to me what is missing is the FX lens circle area over the DX sensor. Would this show the crop factor??? Please advise as this is a personal epiphany
     
  14. bmm

    bmm

    Ed and others - Michael's diagrams are an ideal way of explaining. Take a lens, say a 50mm. With the lens you take an image of an object - say a pencil - that is 10 high in terms of the image the lens produces. This dimension does not change regardless of whether FX or DX is used. However.... The 10 high image of the object on FX fills just under 1/2 of the total 23.9 height of the FX sensor. So it seems comparably smaller, or further away in the image, as if a wider lens was used. The SAME 10 high image of the object on DX fills well on 2/3 of the total 15.8 height of the DX sensor. So it seems comparably larger, or closer in the final image, as if a tighter lens was used. So you see, the image projected by the lens is identical in all respects. What changes is that FX takes and uses a different proportion of the image than DX does. The metaphor that a friend used to explain it to me was to make me imagine a slide projector set up, and two rectangles of white sheet of different sizes. FX is like having a larger sheet which 'captures' most of the projected image. DX is like having a smaller sheet that allows 1/3 of the image to 'fall off' the edge (and so only shows the middle part of the image). The projected image itself doesn't change in any respect.
     
  15. I just take half the given focal length and add it to that focal length to determine the equivalent; so my 12-24 Tokina is 18-36 (6 added to 12 and 12 added to 24). My 24-85 D Nikon is 36-127.5 (12 added to 24 and 42.5 added 85, my 70-300 VR is 105-450 (35 added to 70 and 150 added to 300).
     
  16. Divide the focal length by two, then multiply by three
     
  17. You do not have to change the numbers on a lens. A 50mm lens is always a 50mm lens no matter what kind of camera it will fit on. The DX sensor is smaller than the FX sensor. So when you take a picture with it you must enlarge it 1.5x to make it the same size as the FX sensor. Imagine you wanted to make a 4x6 print. The DX sensor image would have to be enlarged 1.5x more then the FX sensor to have a final outcome image on the 4x6. It gives a telephoto type effect but not in actuality becase it's not a telephoto effect. It is an enlarging the file size effect. However there is really no cropping actually occurring. Nothing is being chopped off. The sensor is just smaller. The 35mm is not a cropped medium format. It's just smaller. The medium format is not a cropped 8x10 because it's just smaller. Here is a great article on DOF effects with the small sensor compared to the larger FX sensor. This becomes more difficult to understand but if you shoot scenics then the 16mm DOF is vast compared to a 24mm lens on a FX sensor. 16mmx1.5=24mm. Image size looks the same but the DOF changes a lot. http://alzodigital.com/online_store/digital_camera_flip_flash_bracket_umbrella_kit.htm
     
  18. The sensor is always "full-frame" (what a misnomer) so nothing is cropped. A lens doesn't know the camera it's attached to so its inherent features such as focal length and captured angle of view don't change (the latter fact is often overlooked). Comparing DX to FX/24x36, the DX sensor is smaller. Depending on how you set up the comparison, with DX you either get a picture with smaller coverage but the same DOF, or the same coverage with greater DOF (because you either stayed put and use a shorter lens, or retreat a few step and use the same lens to get the identical coverage). So the setup will dictate the results and they can be predicted a priori. When you put a lens designed for FX/24x36 on a DX camera, the lens will project a larger image circle than the DX imager can record. So a lot of light rays entering the mirror chamber have no way to go since they don't constitute image-forming rays. This will lead to increased probability of flare and can be a real problem in particular for close-up work and photomacrography. Image contrast can suffer as well. A DX lens has a reduced image circle hence suits the smaller format better. Deep sigh: Comparing different formats usually ends up in the conclusion that the formats are different. A perfect example of circular reasoning. Rethink to utilise the advantages of any particular format instead.
     
  19. Thread hijack ... Bjorn, see this question I asked back in May14th... http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00PU0E ... Does my 35 1.4 AIS 'lack of contrast' at 1.4, 1.8 and 2.0 and which is fine from 2.8 and smaller on a D200, due to the thing you discuss? I had a feeling all that extra light in there, regardless of baffling, black walls and such still amounted to flare potential (maybe flare isn't the correct word for reduced contrast?) I guess too that light can bounce back and forth off the rear element which must lower contras?. Jim M.
     
  20. If I put a FX lens or and older 35mm film camera lens on my D60, (DX Sensor), What do I see in my viewfinder? Do I see what the sensor records -- the croped image, or do I see the uncroped image?
     
  21. The answer is: look in the viewfinder of the camera. With either a DX or a FX camera, the viewfinder shows what portion of the imaging circle of the lens is recorded by the sensor. All images are "cropped" (recorded image format is a rectangle, imaging circle is - well - circular), *unless* there is a severe mismatch (small imaging circle on a big format, for example, a DX lens on a 6x6 camera). So the term "cropped" doesn't mean much if anything at all. Same with "crop factor". If you put an FX lens on the D3, the "factor" is 1. If the D3 is set to 5:4 format, the "factor" is 1.2(horizontal), 1 (vertical) or 1.1 (diagonal). So what is the "crop factor"?? Should the reference format be 4x5" (5x4" for our UK friends) instead? Then the crop factor would unequivocally be 2.5. The point here is that "crop factor" is not an inherent property of the lens and not even has a single-valued interpretation when aspect ratios of the format(s) differ. All these terms can contribute is added confusion. In the large-format days, we thought in terms of covering power, picture angles, and occasionally, had to take the optical design into account since a true telephoto lens will have far less covering power than a non-telephoto lens of the same focal length. Typically you could use lenses designed for the 4x5" format on a 6x9(cm) camera, but the other way around needed careful consideration of the covering power (=one put the lens on the camera and looked at the ground glass for dark corners). Even the covering power wouldn't be a constant since it would for most lenses increase when the lens was stopped down. So we had to use the view finder/ground glass to decide what worked and what didn't. Wish that lesson survived to the current crop of photographers.
     
  22. This isn't high math. When you look in your DSLR viewfinder, whether it is an FX or DX lens, forget all the formulas. What you see is essentially what you get.. Some of the confusion comes from the fact that even on DX lenses they use the same focal length designations used on FX lenses. So an 18mm DX lens is only going to see what a 27mm FX lens would see on a 35mm film camera (or on a "full frame" DSLR like the D3). As stated earlier, the find out what the coverage of an FX lens will be on a DX camera, simply divide the stated focal length of the lens by 2 and then multiply by 3. So, when I put my 20mm f2.8 Nikkor on my D300, divide 20 by 2 = 10, then multiply 10 x 3 =30. So on the D300 the 20mm lens sees as much as a 30mm len would see on a full frame (24x36) camera. The 20mm lens still is a 20mm lens, but the D300 only is using 2/3 of the potential coverage area. But when all is said and done the ONLY thing that really matters is what you see in your viewfinder---what you see is what you get.
     
  23. I updated the example image I posted earlier, hopefully it explains things better.
    00PxiH-52073584.jpg
     
  24. It seems to me what is missing is the FX lens circle area over the DX sensor. Would this show the crop factor???
    People keep telling you, but you just aren't listening. The crop factor has nothing to do with the lens, and therefore nothing to do with the image circle. The crop factor exists because the sensor is smaller. Period.
     
  25. The crop factor thing is a misnomer. The DX is full frame the same as all sensors and formats. (possibly a camera like the D3 is different because it does have a portrait crop feature) If you put a 18mm lens on a DX sensor you do not need to do simple math to figure it out. Just look into the viewfinder and if you like what you see then push the button. It's still an 18mm lens and remains so no matter what kind of camera it will fit on . Yes the field of view and the DOF is different then the 35mm and the medium format and the View camera. It's also different then a 110 camera or a cell phone etc, etc. That because it is not the same format size as those other camera's.
     
  26. It's true that it does not matter if you use an FX lens or a DX lens on a DX camera, just know that the amount of the scene a DX sensor will 'see' is less than the amount of the same 'scene' the FX sensor will see. That's actually what I was trying to present with the last sample I posted. I see now that I should not have shown the FX lens area outside the 1.5 crop area overlays, and left out 'FX lens on a DX sensor'. Here is an update.
    00PyEf-52271884.jpg
     
  27. Does this diagram help? No numbers, but a different perspective.
    00PyFW-52275684.jpg
     
  28. Thanks Michael Kohan for all your explainations and the samples you posted. Its really not easy to understand the whole thing. But guess, I got the thing with FX/DX and the cropping factor. But still I dont get it, what the "Full frame" now means and changes something. I'm working with a D200, and I'm eying to change it into a D3. And to make it easier, lets say I'm going to use only DX lenses, what would be the difference between both cameras?
     
  29. "lets say I'm going to use only DX lenses, what would be the difference between both cameras?" You would need to purchase new lenses, or be restricted to using the D3 in DX mode which will give you a very expensive 5 MPix camera.
     
  30. One can go back 50 or 60 years ago and take a 50mm lens from a Leica thread mount camera; and mount it one ones 16mm Bolex; or 8mm Bolex cine camera. One still is using say a 50mm F2 Summicron; its still a 50mm focal length. When the 50mm lens is on the 35mm still camera; it covers more angle than the smaller 16mm or 8mm cine formats. Sadly this was not rocket science in the slide rule era; but grade school trig. One often used a circular paper slide rule to figure coverage; or ones click stop finder on a cine camera; or the cinematogaphers manual if a pro cine chap. Its amazing the the crop factor term is so confusing; maybe sticking to angular coverage should be retaught. In cine work say for a 16mm camera the normal lens is longer than the diagonal; typically a 25mm lens. For 8mm cine its a 12.5mm lens; ie about 1/2 inch . In the old days folks made and carried 3x5 filling cards with a matrix of angular coverage; or carried a directors sight; and learned a feel for coverage for each format. If this was dog.net would it be confusing to understand that a smaller dog eats less; or would there be a crop factor term?:)
     
  31. With a cellphone camera there is little confusion. Most all dont know the lens focal length or sensor size; thus the angular coverage of the system is what folks see in the viewfinder.
     

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