DX Crop Factor

SkvLTD

Senior Member
Finally read up on what it really means, and so I want to ask this- if typically 85mm is considered a portrait lens, it is so for the angle of view/distortion it offers. So if a 50mm on a DX sensor offers ~75mm FX equivalent field of view, it might as well be that ~85mm equivalent, correct?

And then going down the line, to get true 50mm style distortion, you have to go with 28 or 35mm on a DX body, and so on.
 

Horoscope Fish

Senior Member
Finally read up on what it really means, and so I want to ask this- if typically 85mm is considered a portrait lens, it is so for the angle of view/distortion it offers. So if a 50mm on a DX sensor offers ~75mm FX equivalent field of view, it might as well be that ~85mm equivalent, correct?

And then going down the line, to get true 50mm style distortion, you have to go with 28 or 35mm on a DX body, and so on.
Where you say, "distortion" I think you might mean, "perspective".

Even so, I believe the answer to your question is, "Yes". For example a 35mm lens on a DX body gives you the same perspective as a 52mm lens would on an FX body.
 
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Eye-level

Banned
I could definitely very well be wrong and probably am but I still think that just because the crop factor on DX makes a 50 a 75 that still doesn't make it a 75 short tele portrait lens....it is just a 50 normal. Remember with a wide angle you will get incredible depth in your photos - roads will go on forever objects will appear to have tons of distance between them. You put on a tele like a 105 and everything in the picture will be scrunched up together.

In another couple of weeks my 85 should arrive and I am going to do some serious comparisons between it and the 50...
 

BackdoorArts

Senior Member
There is a "feel" to every focal length, particularly how it deals with perceptive space. A 35mm will tend to distort left-to-right and top-to-bottom slightly. So, when used to take a photo of a face it may make someone's face look abnormally wide if taken fairly close. The same face, photographed with a 50mm lens in a way that it occupies the same percentage of the field of view (i.e. taken from slightly farther away) will exhibit less of this distortion. Similarly the depth of field of the 35mm will be appear to be bigger than the 50mm. For example, an object behind the subject will appear further away from the subject in a photo taken with the 35mm than with the 50mm.

Now, the 35mm on DX camera has an "equivalent focal length" to a 50mm on an FX camera. BUT, this does not change the optical characteristics of the lens. Any optical characteristics will remain intact regardless of the sensor, so the distortion you get from a 35mm lens on an FX camera will be identical to the distortion on a DX camera, but you'll only get the center 42% of the image.

Take a look at this photo, taken with a wide angle (borrowed from Nikon DX vs FX)...

Nikon-DX-vs-FX.jpg


The red box represents the size of the DX sensor while the full photo represents the size of an FX sensor. The difference between the two can be considered wasted projection (my term) from the lens - things the lens sees that the camera cannot. If this were taken with a 35mm lens, were you to put a 50mm lens on an FX body then the red box would represent (approximately) how much of the photo you could capture with the lens, but I suspect that the look of that image would change because of the optical characteristics of the lens.

Maybe this is a better example. Look at the optical characteristics of the photo as the focal length changes...

focal-length-comparison.jpg


So be careful, because you're dealing only with focal length equivalency and not any of the other optical considerations.
 
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§am

Senior Member
To be honest I think if you're never gonna move off DX, the whole crop factor doesn't really play into much overall.
Yes if you had a DX & FX next to each other and used the same lens you'd notice the difference, but having one or the other, are you really going to know?

OK, so you 'need' 85mm for the perfect portrait lens, but isn't that all based on the old film days.
In terms of DX, you could easily just say, you need a 50mm lens to be the perfect portrait lens (equivalent to 75mm which is close enough) and then there's less confusion all round :)
 

Krs_2007

Senior Member
I could definitely very well be wrong and probably am but I still think that just because the crop factor on DX makes a 50 a 75 that still doesn't make it a 75 short tele portrait lens....it is just a 50 normal. Remember with a wide angle you will get incredible depth in your photos - roads will go on forever objects will appear to have tons of distance between them. You put on a tele like a 105 and everything in the picture will be scrunched up together.

In another couple of weeks my 85 should arrive and I am going to do some serious comparisons between it and the 50...

Is that the 85g or d your waiting on. I am having a hard time finding an 85g. Sorry to side swipe the conversation here, but I feel for the
DX
format the 85 would be nice to have.


 

Eye-level

Banned
OK, so you 'need' 85mm for the perfect portrait lens, but isn't that all based on the old film days.

No it is based upon the 24X36 135 format...still extremely relevant to modern digital cameras...this is something I wish they would change...come up with a new even bigger format especially made for digital and modern glass please and make it cheap!
 

BackdoorArts

Senior Member
For portraits there seem to be people in the 85mm camp and the 105mm camp. Looking at the facial features of the center column in my post above, I would say that anything below 50mm adds unnatural distortion making it unusable as a general portrait lens. You could probably get away with a 50mm, which when compared to the 85mm produces comparatively similar looks to the girl's face. So depending on the size of your studio, a DX with a 50mm or an 85mm would work well, much like an 85mm or 105mm would be perfect for a FX camera user. Anything longer than that and you start to lose front-to-back dimensionality of the subject's head. Having a paired set of lenses would likely work best.
 

Krs_2007

Senior Member
That was a great post, it helped me out as well. I was thinking about the 105 but quickly realized the 85 would be better based on that image. At least for myself. Also thinking about going FX, so the 85 would serve its purpose if I do make the switch.

thanks again for that image comparison BackdoorHippie
 

Eye-level

Banned
In FX the general rule is 85's are for half length body portraits and not to be used for head and shoulder shots because the 85 is to "normal" thereby less optically flattering...head and shoulders only is where the 105 comes in...

IMO I think the rule works the same exact way for DX but you have to stand farther away which is a PITA which is why I want a FX camera. I can't get a FX camera right now so I am going to "cheat" and use the 85 (even for head and shoulders) Confused now? Good...me too....LOL
 
There is a "feel" to every focal length, particularly how it deals with perceptive space. A 35mm will tend to distort left-to-right and top-to-bottom slightly. So, when used to take a photo of a face it may make someone's face look abnormally wide if taken fairly close. The same face, photographed with a 50mm lens in a way that it occupies the same percentage of the field of view (i.e. taken from slightly farther away) will exhibit less of this distortion. Similarly the depth of field of the 35mm will be appear to be bigger than the 50mm. For example, an object behind the subject will appear further away from the subject in a photo taken with the 35mm than with the 50mm.

Now, the 35mm on DX camera has an "equivalent focal length" to a 50mm on an FX camera. BUT, this does not change the optical characteristics of the lens. Any optical characteristics will remain intact regardless of the sensor, so the distortion you get from a 35mm lens on an FX camera will be identical to the distortion on a DX camera, but you'll only get the center 42% of the image.

Take a look at this photo, taken with a wide angle (borrowed from Nikon DX vs FX)...

Nikon-DX-vs-FX.jpg


The red box represents the size of the DX sensor while the full photo represents the size of an FX sensor. The difference between the two can be considered wasted projection (my term) from the lens - things the lens sees that the camera cannot. If this were taken with a 35mm lens, were you to put a 50mm lens on an FX body then the red box would represent (approximately) how much of the photo you could capture with the lens, but I suspect that the look of that image would change because of the optical characteristics of the lens.

Maybe this is a better example. Look at the optical characteristics of the photo as the focal length changes...

focal-length-comparison.jpg


So be careful, because you're dealing only with focal length equivalency and not any of the other optical considerations.

This is an aspect I had not thought about before since I really don't do portraits yet. But I would have incorrectly assumed that the 50mm would ACT like a 75mm . I would have been wrong.
 

Krs_2007

Senior Member
In FX the general rule is 85's are for half length body portraits and not to be used for head and shoulder shots because the 85 is to "normal" thereby less optically flattering...head and shoulders only is where the 105 comes in...

IMO I think the rule works the same exact way for DX but you have to stand farther away which is a PITA which is why I want a FX camera. I can't get a FX camera right now so I am going to "cheat" and use the 85 (even for head and shoulders) Confused now? Good...me too....LOL

Nope, just sitting here spinning in my chair.... I hear ya on the FX and cant get it off my mind, but forcing myself to hold off a bit.
 

SkvLTD

Senior Member
So to sum it up- same lens will essentially look more zoomed in on DX than FX by having a lesser angle of view, but the perspective will not change since the way the light hits the sensor does not change, right? Thus all that A on DX is equivalent of B on FX only has to do with the field of view/zoom?
 

Marcel

Happily retired
Staff member
Super Mod
So to sum it up- same lens will essentially look more zoomed in on DX than FX by having a lesser angle of view, but the perspective will not change since the way the light hits the sensor does not change, right? Thus all that A on DX is equivalent of B on FX only has to do with the field of view/zoom?

You are absolutely right. The word crop says it all. It's exactly like if you took an 8x10 print and just masked the outside to make it 1.5 times smaller. The perspective does not change, but the image content does.
 

SkvLTD

Senior Member
Guess it might be more liberating whenevernever(lol) I go into FX, but for now I'd either have to compensate perspective for field of view or simply deal with it. lol
 

SkvLTD

Senior Member
I was after a better low-light glass/a prime for simplicity and well, though I don't like 50 as much as a length, price point certainly won. Now all I've left is a 70-300 of sorts and I'll have my trinity.
 
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