rocketman122
Senior Member
I read a comment on DPreview saying something about the DOF with FF not looking the same as DX. what does this mean? can someone explain it?
thank you.
thank you.
read a comment somewhere saying about DOF FF vs DX. can someone explain this?
- Thread starter rocketman122
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BackdoorArts
Senior Member
Find any one of a bunch of FX/DX threads here and you're bound to find photo examples I've posted (no time to dig for you today).
Reader's Digest version, there is absolutely no difference in DoF when you put the same lens on a DX or FX camera. Where there is a difference is when you try and match perspectives. With a 50mm lens on an FX camera you will need a 35mm lens on a DX camera in order to capture the same image view since the DX sensor is 1.5x smaller (i.e. smaller sensor needs a wider angle lens to see the same thing the larger sensor sees). A 35mm lens produces different DoF than a 50mm lens, so there will be an apparent difference in DoF with photos taken on a DX and FX camera, but it's because you're using different lenses.
Reader's Digest version, there is absolutely no difference in DoF when you put the same lens on a DX or FX camera. Where there is a difference is when you try and match perspectives. With a 50mm lens on an FX camera you will need a 35mm lens on a DX camera in order to capture the same image view since the DX sensor is 1.5x smaller (i.e. smaller sensor needs a wider angle lens to see the same thing the larger sensor sees). A 35mm lens produces different DoF than a 50mm lens, so there will be an apparent difference in DoF with photos taken on a DX and FX camera, but it's because you're using different lenses.
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rocketman122
Senior Member
wow, quick reply. yes, I understand that the focal length plays a role. meaning a 100mm lens on a FF, and to get the same DOF and perspective (to make both pictures from the cameras look the same) you would use a 75mm on a DX? so if that parameter is the same then there is no difference with DOF between them? I understand there are much more than numbers of focal length here and different quality lenses but if the equivalent focal length is used on both then no difference, right? thank you.
BackdoorArts
Senior Member
It would actually be a 66mm lens on a DX (to get from DX to FX divide by 2 and add that to the original focal length).
There are 2 ways of looking at the question...
Method 1: Both cameras have the same lens, so in order to take the exact same photograph you have to stand further away with the DX camera than you would with the FX camera. The increased distance from the subject also increases your depth of field...
Method 2: You stand in the same place and use different lenses on each camera to frame the subject the same (i.e. 35mm on a DX, 50mm on an FX). The different lenses will produce different DoF at the same apertures.
There are 2 ways of looking at the question...
Method 1: Both cameras have the same lens, so in order to take the exact same photograph you have to stand further away with the DX camera than you would with the FX camera. The increased distance from the subject also increases your depth of field...
Method 2: You stand in the same place and use different lenses on each camera to frame the subject the same (i.e. 35mm on a DX, 50mm on an FX). The different lenses will produce different DoF at the same apertures.
BackdoorArts
Senior Member
One thing I failed to point to in the second photo above, when using method #2 the difference in DoF is usually about 1 stop. Compare 85mm at f2.8 and 135mm at f4 - they are pretty close.
rocketman122
Senior Member
Very nice examples. But I thought the dof would be the same because the focal length was equivalent (more or less) but i see it simply isn't so. Is it because the 135 lens property and the way its built that simply has a more blurred bokeh? Btw, is it the 135dc in those pics?
if we took sigmas 70mm for dx and the 105mm they have for the ff would the dof be the same?
you said the dof should be the same if the lens had the equivalent focal length but like you said I noticed its aprx one stop in dof (with the pics shown above) and that's a lot. One stop can mean the success or failure of a picture at times.
if we took sigmas 70mm for dx and the 105mm they have for the ff would the dof be the same?
you said the dof should be the same if the lens had the equivalent focal length but like you said I noticed its aprx one stop in dof (with the pics shown above) and that's a lot. One stop can mean the success or failure of a picture at times.
BackdoorArts
Senior Member
It's all about optics, and there are plenty of great blogs that outline exactly how focal length and dof interact - just get out your Google and Bing fingers and start typing.
I never said anything about equivalent focal length and DoF being the same. There is no such thing as "focal length equivalent", only "framing equivalent". 35mm DX is not equivalent to 50mm FX, but they will give you a similarly framed photo. 85mm DX is not equivalent to 135mm FX, but they will give you a similarly framed photo. 35mm is 35mm is 35mm, regardless of format - the only difference is the size of the sensor behind it and how much of the image produced by that lens gets captured.
One lens, one projection, two different size captures. You can never - NEVER - get the exact same photo from a DX and FX camera (without cropping the FX photo to DX size) because every aspect of an image changes with focal length. Perspective changes in the field of view as it changes, which is why faces shot with a wide angle look warped compared to those taken with a portrait lens when you move to frame the same photo. The apparent distance front to back changes as focal length changes. This is a great photo illustrating that how focal length alone changes those things...
Seeing these two concepts it should be immediately obvious that there is no such thing as focal length equivalency, because the minute you change that something is going to be altered that you cannot reproduce with any other focal length without moving the stuff in your viewfinder around.
So, to answer your specific question, "if we took sigmas 70mm for dx and the 105mm they have for the ff would the dof be the same?" No!! At least not at the same aperture value. The 70mm at f2.8 on a DX camera would have roughly the same dof as the 105mm at f4 on an FX camera, but other aspects of the photo will be different.
DX and FX are different beasts capable of producing more than similar photos - but never the same photo. But comparing one to the other without fully understanding the basics behind each is like comparing apples to squash tournaments. OK, maybe not that bad, but people need to stop thinking about how you make one like the other and start thinking about what each one is on its own. It's not that difficult a concept, and as I said there are plenty of detailed explanations out there - most of which I've linked here at one time or another. Learn about how focal length impacts Depth of Field, independent of format (FORGET FORMAT!!), and every other aspect of the image. It will help you decide which lens to use on that portrait so you don't step close to a model with a 28mm - she'll be pretty pissed when her thin face looks pudgy.
I never said anything about equivalent focal length and DoF being the same. There is no such thing as "focal length equivalent", only "framing equivalent". 35mm DX is not equivalent to 50mm FX, but they will give you a similarly framed photo. 85mm DX is not equivalent to 135mm FX, but they will give you a similarly framed photo. 35mm is 35mm is 35mm, regardless of format - the only difference is the size of the sensor behind it and how much of the image produced by that lens gets captured.
One lens, one projection, two different size captures. You can never - NEVER - get the exact same photo from a DX and FX camera (without cropping the FX photo to DX size) because every aspect of an image changes with focal length. Perspective changes in the field of view as it changes, which is why faces shot with a wide angle look warped compared to those taken with a portrait lens when you move to frame the same photo. The apparent distance front to back changes as focal length changes. This is a great photo illustrating that how focal length alone changes those things...
Seeing these two concepts it should be immediately obvious that there is no such thing as focal length equivalency, because the minute you change that something is going to be altered that you cannot reproduce with any other focal length without moving the stuff in your viewfinder around.
So, to answer your specific question, "if we took sigmas 70mm for dx and the 105mm they have for the ff would the dof be the same?" No!! At least not at the same aperture value. The 70mm at f2.8 on a DX camera would have roughly the same dof as the 105mm at f4 on an FX camera, but other aspects of the photo will be different.
DX and FX are different beasts capable of producing more than similar photos - but never the same photo. But comparing one to the other without fully understanding the basics behind each is like comparing apples to squash tournaments. OK, maybe not that bad, but people need to stop thinking about how you make one like the other and start thinking about what each one is on its own. It's not that difficult a concept, and as I said there are plenty of detailed explanations out there - most of which I've linked here at one time or another. Learn about how focal length impacts Depth of Field, independent of format (FORGET FORMAT!!), and every other aspect of the image. It will help you decide which lens to use on that portrait so you don't step close to a model with a 28mm - she'll be pretty pissed when her thin face looks pudgy.
BackdoorArts
Senior Member
(dammit - I need to just blog on this so I can just link to that from now on!! lol)
rocketman122
Senior Member
Good stuff thanks for your time and effort BDH!