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12-04-2020, 09:19 AM   #46
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QuoteOriginally posted by stevebrot Quote
For some reason I tend to do a fair amount of gear photos and have found of late that I have not been taking sufficient care to not get too close. In the photo below, I inadvertently "bent" my new Samyang 14mm a little. Changing from a 35mm to a 50mm lens probably would have lessened the apparent "bend" quite a bit.
That's a good example of how perspective distortion at close range exaggerates elements near the camera. I guess if you view it from a great enough distance, the distortion should cancel out

[Nice photo, too The Samyang looks great on your KX, and the XR Rikenon 35/2.8 seems sharp and smooth in equal measure ]

For equipment shots, I prefer to shoot roughly a couple of metres (6 - 7') from the item, though I'm not precise about it. I've a bunch of shots like yours where I was too close and exaggerated the angles to an unnatural extent. I've not always been able to correct them satisfactorily in post-processing either

12-04-2020, 09:57 AM   #47
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QuoteOriginally posted by Digitalis Quote
FYI: these were taken on a full format DSLR, taken at a fixed aperture of f/2.8
I would stress that these images depend on the fact that the F-number was fixed, while the aperture (size) was changed massively along the focal lengths.

---------- Post added 4th Dec 2020 at 18:00 ----------

QuoteOriginally posted by pschlute Quote
For many years I believed that a longer FL had an inherent property of being able to deliver a narrower DOF. So if I wanted a head and shoulders shot and wanted the narrowest DOF I would zoom out to maximum FL right ? Wrong. It was quite a revelation when I learned years later that this was not the case. For instances where the subject size is kept constant in the viewfinder/film/sensor, the DOF is identical across all FL (except for very close macro)
Given a constant aperture size and same subject framing the longer FL actually gives you linearly more/deeper DoF at the exact same distant background blur/separation.
12-04-2020, 10:12 AM   #48
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QuoteOriginally posted by beholder3 Quote
Given a constant aperture size and same subject framing the longer FL actually gives you linearly more/deeper DoF at the exact same distant background blur/separation.
Keeping a constant aperture size while zooming would involve me using my calculator to work out the f-stop and I suspect the subject would get bored and move away
12-04-2020, 11:04 AM - 1 Like   #49
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QuoteOriginally posted by beholder3 Quote
Given a constant aperture size and same subject framing the longer FL actually gives you linearly more/deeper DoF at the exact same distant background blur/separation.
This is where I say...show me.

Edit: I still think a demonstration is helpful. Beholder3's assertion has a couple of loose ends that are not worth pursuing here. A new thread showing how this works would be very informative (not being sarcastic). I did this test several years ago, but maybe not the same way that they envision.


Steve


Last edited by stevebrot; 12-04-2020 at 07:32 PM. Reason: grammar and clarity
12-04-2020, 04:26 PM - 1 Like   #50
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QuoteOriginally posted by beholder3 Quote
I would stress that these images depend on the fact that the F-number was fixed, while the aperture (size) was changed massively along the focal lengths.
They say you are never too old to learn. Having done photography now for over 40 years I guess I should have been aware of this but I had never given it any thought. My 24-70 zoom lens is a constant f2.8. But as I zoom from 24mm to 70mm, the aperture size increases. Logical of course, as the f number is a ratio of FL to aperture size and so it must.

---------- Post added 12-04-20 at 11:38 PM ----------

QuoteOriginally posted by stevebrot Quote
This is where I say...show me.
Steve, note he is referring here to aperture "size" not f- number. The same f number must give a different aperture "size" at each FL setting or the ratio would not hold true.

To keep the same aperture "size" as you zoom to a longer focal length, you would need to stop down. ...ie a zoom : at 50mm at f2.8 it has an opening 17.85mm diameter. At 100mm that zoom needs to be at f5.6 for the aperture to maintain a 17.85mm diameter

I think

---------- Post added 12-05-20 at 12:01 AM ----------

QuoteOriginally posted by beholder3 Quote
Given a constant aperture size and same subject framing the longer FL actually gives you linearly more/deeper DoF at the exact same distant background blur/separation.
That is because you are stopping down the lens !

Last edited by pschlute; 12-04-2020 at 04:39 PM.
12-04-2020, 06:06 PM   #51
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QuoteOriginally posted by normhead Quote
It always amazes me when people use words to describe readily demonstrable visual phenomena. Why do they do that?
shrug It's a great way to pad an essay or lengthy treatise.
12-04-2020, 06:20 PM   #52
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QuoteOriginally posted by pschlute Quote
I think
That is why a demonstration is preferable. I may learn something.


Steve

12-05-2020, 03:39 AM   #53
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QuoteOriginally posted by pschlute Quote
Keeping a constant aperture size while zooming would involve me using my calculator to work out the f-stop and I suspect the subject would get bored and move away
I am not discussing the different variants of how one can approach making different photos or "zooming".
Think of comparing a 50/1.4 lens versus a 200/5.6 lens wide open for a portrait on a corn field.


QuoteOriginally posted by stevebrot Quote
This is where I say...show me.
Anyone can compare any situations he wants here: DOF simulator - Camera depth of field calculator with visual background blur and bokeh simulation.
Use Framing: Lock field of view for a constant portrait framing.
On the upper right corner you see the background blur, e.g. 1.880 mm for both.

On the lower end you see the DoF; 12.4cm vs. 3.1 cm


So you get the same subject separation, but the DoF is 4x bigger on the 4x longer FL.

1)

Essentially this means that if you want good subject separation but still want nose tip and ears sharp and have enough space and the background is far away, you'll want to use 200mm F5.6 and not 50mm F1.4. Large aperture size is your friend in this scenario.


The sharpness drop off due to DoF is much more aggressive for a lens with smaller F-number and same aperture size.

2)

On the other practical side end, if you shoot in a small space and nasty background is very near to the subject, your best friend is sheer F-number. You'll want to use a F1.2 lens.
12-05-2020, 04:01 AM - 2 Likes   #54
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Most people choose a focal length based on what they plan to shoot and the working distance they will have. 85mm on full frame or 55mm on APS-C are traditional portrait lengths because of the working distance involved there. Using a 200mm or 300mm might work in a time of COVID, but you'll be standing far enough back that it will be harder to maintain a connection with your subject.

Depth of field is immaterial to this whole discussion and will differ greatly depending on the focal length, aperture, and cropping.
12-05-2020, 09:39 AM   #55
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QuoteOriginally posted by beholder3 Quote
Anyone can compare any situations he wants here: DOF simulator - Camera depth of field calculator with visual background blur and bokeh simulation.
That is a simulator. I would suggest a real world subject and 35mm vs 50mm lens. The 50mm at f/4 will have the same size aperture opening as the 35mm at f/2.8. It should be easy to set up and if you get more/deeper DOF with the longer focal length, it should be obvious by adding a ruler to the setup.

Optical diagrams as well as the math suggest the DOF should be the same*, but I have been wrong before in my interpretation of such things.


Steve

* Assuming same subject distance and same final magnification

Last edited by stevebrot; 12-05-2020 at 10:30 AM.
12-05-2020, 10:36 AM   #56
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QuoteOriginally posted by beholder3 Quote
Use Framing: Lock field of view for a constant portrait framing.
The simulator changes subject distance if you lock field of view.


Steve
12-05-2020, 08:01 PM   #57
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QuoteOriginally posted by normhead Quote
Were you stoned when you wrote that?
Nope I just know that there is no DOF until we first decide how we are going to view it, If we don't hold how we are going to view it then we can say all kind of stoned and silly things like if we shoot a 100mm @ ƒ2.8 at 2m distance to the subject we can have the same DOF using that same lens shot on the same format, at ƒ5.6 and at the same distance we can will have the same DOF.
12-05-2020, 08:24 PM - 3 Likes   #58
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12-05-2020, 08:52 PM   #59
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QuoteOriginally posted by Ian Stuart Forsyth Quote
I just know that there is no DOF until we first decide how we are going to view it
Yep, there is only one point of focus and it is infinitesimally small. DOF is our perception of objects at that point.


Steve
12-05-2020, 08:59 PM   #60
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QuoteOriginally posted by Not a Number Quote
Very good!


Steve
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