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09-17-2014, 07:45 PM   #1
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DoF 135mm 2.8 vs 85mm 1.4

Hello,

I'm looking for a lens that will help me bring my portrait work to the next level. I would like to work with the thinnest possible depth of field. I have the 50-135 f2.8 which is a great lens, although a little tricky to use at 135mm and 2.8.
I was wondering if the Sigma 85mm used wide open at f1.4 would help me capture a thinnest DoF than the 50-135 at 135mm and 2.8... ?
My second question: is the FA 77 so special with the way it renders that ultimate DOF control would be of secondary importance with this lens.

I would keep the 50-135 anyway for its flexibility and WR....

Thanks!

09-17-2014, 08:08 PM   #2
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A 135mm @ F/2.8 distance 10 ft. DoF = .18 ft.

85mm @ F/1.4 distance 10 ft. DoF = .23 ft.

But of course it's not that simple as FoV and perspective are completely different between the 2 focal lengths.
09-17-2014, 08:12 PM   #3
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QuoteOriginally posted by boriscleto Quote
A 135mm @ F/2.8 distance 10 ft. DoF = .18 ft.

85mm @ F/1.4 distance 10 ft. DoF = .23 ft.
Thank you.

Is there a way to calculate this for 2 pictures with the same composition, and not a fixed 10 ft.?
09-17-2014, 08:16 PM   #4
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DOF gets thinner the closer you are to the subject, the greater the focal length, and the larger the aperture. For a given subject framing, the distance to the subject and the focal length are directly inverse, so they cancel each other out, leaving only aperture. If you want a thin depth of field, and don't care about other factors, then you only need to worry about aperture.

All that said, you really don't want the "thinnest DOF possible". The problem you run in to is the picture looks all blurry, but you notice that one little tiny part of the subject is in focus. If you want to experiment, just get an old M50/1.4 It's really a great lens, and has a super fast aperture. Then you can decide where you want to go from there.

09-17-2014, 08:20 PM - 1 Like   #5
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Evening, I really don't do shallow DoF, but I can do the math. The 85 @ f1.4 will have the thinnest DoF when you compare both lenses at a distance that will produce the same frame size. What I did was just take the 85 and selected a 10 foot shooting distance (with a crop of 1.53) and came up with a frame coverage of 2' 9.2" x 1' 10.1" (diagonal of 3' 3.9"). Then using the first tool below, and using 135mm I found the distance that produced the same frame size - 15.9 feet.Then I used the DOF calculator below to determine the DOF for each lens, wide open at the the appropriate distance. The results are.....
  • 85/f1.4 @ 10 feet has a DoF of 0.23 feet
  • 135/f2.8 @ 15.9 feet has a DoF of 0.47 feet
Even with the physical size of the frame being pretty equal, I would think that you would have some inherent difference in the image caused by the focal length of the lens and how it "sees" the person being photographed. The longer the focal length the more natural looking the image, especially when compared to the ultra wide angle focal lengths.

09-17-2014, 08:20 PM   #6
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QuoteOriginally posted by Kozlok Quote
DOF gets thinner the closer you are to the subject, the greater the focal length, and the larger the aperture. For a given subject framing, the distance to the subject and the focal length are directly inverse, so they cancel each other out, leaving only aperture. If you want a thin depth of field, and don't care about other factors, then you only need to worry about aperture.
I didn't realize this... Thank you.

---------- Post added 09-17-14 at 10:23 PM ----------

Thanks interested observer!
09-17-2014, 08:59 PM   #7
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If you ask me 50mm is a pretty good sweet spot on aps-c because it works out to 75 which is a short telephoto. That said as the above picture shows, there exists a sweet spot between 85 and 135mm where the perspective gives the most flattering appearance. For head shots on aps-c 50-85mm is probably best. I really like the 70-80ish FL personally because you can start to get some nice isolation from the background. Of course a 200mm will give you that too. BTW, wide angle lenses can be ok if you keep your subject in the center where distortion is lowest. You can get some interesting shots with them. I think the above example is a bit exaggerated below 50mm. Get a 50 1.4. That will tell you if you need less dof. If that's the case, start looking at the samyang 85mm or if you want to blow some cash find one of the many classic pentax 85mm lenses. The K85/1.8 is currently undervalued and share's the same formula as the better of the Tak 85s, None of them are as cheap as the samyang though.

09-17-2014, 09:09 PM   #8
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QuoteOriginally posted by Kozlok Quote
DOF gets thinner the closer you are to the subject, the greater the focal length, and the larger the aperture. For a given subject framing, the distance to the subject and the focal length are directly inverse, so they cancel each other out, leaving only aperture. If you want a thin depth of field, and don't care about other factors, then you only need to worry about aperture.

All that said, you really don't want the "thinnest DOF possible". The problem you run in to is the picture looks all blurry, but you notice that one little tiny part of the subject is in focus. If you want to experiment, just get an old M50/1.4 It's really a great lens, and has a super fast aperture. Then you can decide where you want to go from there.
nicoprod - As Kozlok observed, you get the thinnest DoF at the closest ranges. However at the closest ranges, you need the wide angle lenses in order to get a person's entire face into the frame - but then it will be distorted. Along with the distortion, the nose will be the only thing in focus. What it boils down to is - everything in moderation. A reasonable distance from the subject to keep a reasonable framing, along with an appropriate aperture selection.

So, with all of that, what is the DoF of a face. Let's say around 12 inches to get everything in - some distance in front of the face to back to the ears. I think what you want to do is to blur the background in order to create some subject separation. In film, it was something like 70 to 135 mm as popular focal lengths. With the smaller sensors, the 70mm essentially turns into 50mm (crop factor). This is great for you, since there are a lot of 50 mm lenses available with fast apertures from f2 to f1.2 available with out costing you your first born.
  • A 50 mm at f2.0 at 10' has a DoF of 0.96 feet
  • A 50 mm at f1.8 at 10' has a DoF of 0.86 feet
  • A 50 mm at f1.7 at 10' has a DoF of 0.81 feet
  • A 50 mm at f1.4 at 10' has a DoF of 0.68 feet
  • A 50 mm at f1.2 at 10' has a DoF of 0.57 feet
If you want a thinner DoF just move closer, however your frame will get accordingly smaller.

50/f2 are easy to find for $20 (with a film camera attached). 50/f1.2 are difficult to find and can be very expensive. Also, don't forget about the older screw mount M42 Tak lenses (with an adapter). They are plentiful, wonderful glass, and are widely available (ebay, craigslist along with the marketplace here on the Forum). Probably the optimal lens for a reasonable price will be a 50/f1.7 to a 50/f1.4.

09-17-2014, 09:12 PM   #9
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Try DoF Master online calculator Online Depth of Field CalculatorThere are smart phone apps as well
09-17-2014, 09:17 PM   #10
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QuoteOriginally posted by interested_observer Quote
The longer the focal length the more natural looking the image, especially when compared to the ultra wide angle focal lengths.
Focal length has nothing to do with it. Distance from the subject has everything to do with it. The differences in the sequence of portraits above is the result of changing perspective due to subject distance from the camera. The 16mm shot if made from the same camera location and cropped to the same framing will have the identical perspective.

Strangely, the DOF will also be the same if the absolute aperture was the same. Hmmm...different rat hole and a deep one at that...

In regards to focal length and portrait photography on APS-C:
  • A reasonable subject distance (say 5 to 8 feet) is far enough to "flatten" perspective. Little is gained by moving further away.
  • Any focal length 50mm or greater will allow that minimum 5 foot subject distance
  • A longer lens (say 135mm) may require that you find a larger room while providing little return in terms of a more flattering perspective
  • Any lens 50mm or longer at f/2.8 or faster will provide sufficiently shallow DOF for selective focus
  • It is fiendishly difficult to manage truly shallow DOF in the portrait setting if critical focus is desired. A good example would be my Jupiter-9 85/2 at 5' for a (very) tight head shot:
    • DOF is only 1 inch @ f/2. Would you like the nose or the eye in focus? You can't have both. Consider too that 1" DOF will not stand up to pixel peeping.
    • F/2.8 will give you 1.3 inches
    • F/1.4 (with the Sigma) takes it down to 0.7 inches...not good. Would you like the eye or the eyebrows in focus? You can't have both.


Steve

---------- Post added 09-17-14 at 09:24 PM ----------

QuoteOriginally posted by nicoprod Quote
My second question: is the FA 77 so special with the way it renders that ultimate DOF control would be of secondary importance with this lens.
I own the FA 77/1.8 and I never really think about ultimate DOF control, even when shooting that lens on film. It is a fine optic with abundant pixie dust and a wide maximum aperture useful for low light and/or selective focus. Mostly I like the pixie dust.

For portraits, I generally use one of my fast 50s or my FA 35/2.


Steve

Last edited by stevebrot; 09-17-2014 at 09:36 PM.
09-17-2014, 09:28 PM   #11
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Try to play here to imagine better.

A.
09-18-2014, 04:28 AM   #12
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Hi OP -- Is your goal super thin depth of field or biggest difference between subject sharpness and background blur? The longer the focal length, the more blurred the background even if the depth of field is identical. Longer lenses make it easier to get a fully in focus face using a slightly smaller aperture and wider depth of field, while at the same time getting an even more blurred out background. The biggest problem with 135 2.8 is working distance.
09-18-2014, 05:40 AM   #13
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QuoteOriginally posted by mgoblue Quote
Hi OP -- Is your goal super thin depth of field or biggest difference between subject sharpness and background blur? The longer the focal length, the more blurred the background even if the depth of field is identical. Longer lenses make it easier to get a fully in focus face using a slightly smaller aperture and wider depth of field, while at the same time getting an even more blurred out background. The biggest problem with 135 2.8 is working distance.
My goal is to blur the background as much as possible. Again I have the 50-135 and I enjoy it a lot, but one thing I didn't mention is that doesn't lock focus at 135mm quite often on the K-3. I also have the FA 50 1.7 which I use for close up portrait with focus on the eyes. I'm a portraitist and I was looking for a 3rd option.
Anyway thanks everybody!
09-18-2014, 08:23 AM   #14
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I've been hoping to do a direct comparison between my 50 1.4, 85 1.4, and 135 2.5 for portraits with the most blur on the same background. I use them differently and I've never actually direct compared. The 85 is my very recent addition. May throw my 30 in as well. Problem is finding the time. Maybe this weekend. I'll try to post my thoughts.

Sorry to hear about your focus issue. I don't have that lens but I find manual focusing 135 at 2.5 pretty easy on the k50. I would think your autofocus should nail it easily. What about in live view? I like manual focus for portraits but that's being old school I know.
09-18-2014, 11:32 AM   #15
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For an example of shallow depth of field I took the picture in this thread: https://www.pentaxforums.com/forums/29-welcomes-introductions/273279-recently-got-k50-broke.html using my manual focus A* 85mm f/1.4 at f/2.8 from a closer than normal distance. The focus to me looks pretty good but as expected the focus is very shallow which is what I was going for.

Again - to be clear - this was NOT a wide open shot - that would have even less focus depth. Using f/2.8 was wide enough to get this effect.
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