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View Poll Results: Best aperture in lens?
f/5.6 (f/8 taking into account the 1.4x tele) 770.00%
f/4 (f/5.6 taking into account the 1.4x tele) 220.00%
Other (please explain) 110.00%
Voters: 10. You may not vote on this poll

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02-24-2019, 05:07 AM   #1
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Best lens aperture (best sharpness) with a tele-converter?

Hi,

I have some doubts about what is the best aperture setting in a lens (best sharpness) when attaching a teleconverter.

Let's write an example and a poll (numbers are hypothetical, just for the example)

Lens: Pentax-A* 400 f/2.8
Best aperture for lens alone (best sharpness): f/5.6
Teleconverter: 1.4x

The question here is whether the best aperture is f/5.6 taking into account the 1.4x multiplier (final aperture f/5.6 = f/4 * 1.4) or not taking it into account (final aperture f/8 = f/5.6 * 1.4)

What is the best aperture for the lens?
* f/5.6 (f/8 taking into account the 1.4x tele)
* f/4 (f/5.6 taking into account the 1.4x tele)
* Other

Please correct me if i'm wrong in any concept I'm writting here, and please explain your answer.

Thanks a lot

02-24-2019, 05:59 AM   #2
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the best aperture with a Teleconverter is the same (similar to Aperture ring on the Lens) you would choose without the Teleconverter. With the Teleconverter the choosen aperture is mathematically multiplied with the converter factor. Which aperture you choose belongs to the fact, if the converter converts the aperture or not. The DA 1.4x HD converter for example automatically shows the right aperture on the settings of the body. (lens f5,6 x 1.4 shows f8 [f 7,84] ). The older converters like the A 1.4x L Converter only shows the aperture you have choosen on the lens (for example lens f 5.6 shows as f5.6) but in reallity it will be f5.6x 1.4. It is just a software thing what the body shows. If the lens best aperture is f 5.6 choose f 5.6 on lens for best resolution.
02-24-2019, 06:50 AM   #3
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Moved to correct forum, Lens discussion, General Photography is not the place for this discussion.
02-24-2019, 07:02 AM   #4
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Thanks pid & photolady.

Regards

02-24-2019, 08:55 AM - 1 Like   #5
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the only comment i would make is while i agree with the above that the best aperture for sharpness is the same as the lens native aperture (i.e. the lens in this case set to F5.6, and assuming the TC does not modify the aperture reported back to the camera set the aperture to F5.6 with the TC,

BUT consider also the end result, i.e. the final exposure solution, any blurr to motion will be magnified by the TC and you have for the lens at F5.6 plus the loss of 1 stop in shutter speed, a slower shooting shutter speed, it may not be sufficient for your subject. image sharpness is more than just what a lens can do, and for me, i prefer to actually bump the ISO a few stops to get a focal length appropriate shooting speed, and accept a sharp image with a little grain, over a blurry one due to motion, with no grain
02-24-2019, 09:15 AM   #6
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Thanks Lowel for the input

Regards
02-24-2019, 12:38 PM   #7
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QuoteOriginally posted by palikrovol Quote

I have some doubts about what is the best aperture setting in a lens (best sharpness) when attaching a teleconverter.

The question here is whether the best aperture is f/5.6 taking into account the 1.4x multiplier (final aperture f/5.6 = f/4 * 1.4) or not taking it into account (final aperture f/8 = f/5.6 * 1.4)

Please correct me if i'm wrong in any concept I'm writting here, and please explain your answer.
a) As others have said, if you find it's f/5.6 without the TC, then it's still f/5.6 with the TC (although the effective amount of light is equivalent to an aperture of f/8). You'd still call that f/5.6.

b) There is enough sample variation and the combination optical physics with a TC, that although "a" is logical in theory, in practice you may find different results depending on the lens, the teleconverter, and the distance at which it is focused. In other words, the factual answer is found in actual testing.
02-24-2019, 01:33 PM - 1 Like   #8
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Thanks a lot for the info Alex.

Regards

02-27-2019, 07:49 AM   #9
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After some thinking I have to disagree - in part. Diffraction kicks in at larger aperture numbers. With digital cameras diffraction often becomes visible at f/8 or even earlier. If f/8 is the sweet spot of your lens, a 2xTC will yield f/16, making diffraction effects more prominent. Therefore, I assume that Optimum aperture is not necessarily the same with or without TC. Diffraction has to be balanced with maximum performance, therefore opening up the aperture with TC installed can yield better results. It all depends on the lens, the camera sensor and the TC.
You need to test it.
02-27-2019, 10:56 AM   #10
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Good point Zapp,

That is the reason I wanted to open this thread, to be aware of this kind of comments.

Thanks a lot.
02-27-2019, 11:56 AM   #11
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QuoteOriginally posted by zapp Quote
After some thinking I have to disagree - in part. Diffraction kicks in at larger aperture numbers. With digital cameras diffraction often becomes visible at f/8 or even earlier. If f/8 is the sweet spot of your lens, a 2xTC will yield f/16, making diffraction effects more prominent. Therefore, I assume that Optimum aperture is not necessarily the same with or without TC. Diffraction has to be balanced with maximum performance, therefore opening up the aperture with TC installed can yield better results.
Counterpoint: Yes, the effective amount of light may become the equivalent of f/16, but the aperture is still f/8 on the lens. The TC does not convert the diffraction of the lens to what that lens would diffract at f/16. Any degradation of the image would be due to the quality of the TC itself. You'll see this if you compare an OEM TC with a third party equivalent.

QuoteOriginally posted by zapp Quote
It all depends on the lens, the camera sensor and the TC.
You need to test it.
Yes, completely agree with this. Theory is theory and is a helpful guide. But "the proof is in the pudding".
02-28-2019, 09:11 AM   #12
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QuoteOriginally posted by zapp Quote
After some thinking I have to disagree - in part. Diffraction kicks in at larger aperture numbers. With digital cameras diffraction often becomes visible at f/8 or even earlier. If f/8 is the sweet spot of your lens, a 2xTC will yield f/16, making diffraction effects more prominent. Therefore, I assume that Optimum aperture is not necessarily the same with or without TC. Diffraction has to be balanced with maximum performance, therefore opening up the aperture with TC installed can yield better results. It all depends on the lens, the camera sensor and the TC.
You need to test it.
QuoteOriginally posted by Alex645 Quote
Counterpoint: Yes, the effective amount of light may become the equivalent of f/16, but the aperture is still f/8 on the lens. The TC does not convert the diffraction of the lens to what that lens would diffract at f/16. Any degradation of the image would be due to the quality of the TC itself. You'll see this if you compare an OEM TC with a third party equivalent.



Yes, completely agree with this. Theory is theory and is a helpful guide. But "the proof is in the pudding".
i think one needs to consider diffraction quite differently, Diffraction is always present, even wide open, diffraction is the bending of light around an edge, in this case the aperture. The issue with diffraction is that as you stop down the lens, the amount of light that is projected to the image which is distorted by difraction, increases as a percentage of the total light emitted from the lens, diffraction is much worse, on shorter lenses because the aperture is relatively small to begin with, and therefore plays a much more important role than on a long lens.

as a result i tend to discount it, for me there are a lot of other causes of lack of image sharpness that occur with very long lenses, especially due to vibration when on a tripod, shake when hand held, image movement due to low shutter speed, distortion due to air currents when shooting over long distances, and focus inaccuracy due to perhaps camera shake or movement to and away from the subject when hand held combined with narrow depth of field.

using long lenses, (beyond 300mm) requires a lot of work, the learning curve for 500mm+ lenses can be quite steep, and there are a lot of people out there who lack the patience and persistence to work outy all the issues. We should not get bent out of shape here discussing the theoretical optimum capabilities of a lens, before we tackle all the practical limits of its use. i for one, dont shoot MTF charts for a hobby
02-28-2019, 09:47 AM   #13
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Without getting all technical

A 300 3.8 lens with a 1.4 aperture so say 400 ƒ4 is functionally the same as a 400 ƒ4 lens. Both will start to become diffraction limited at ƒ5.6.

There's no difference between adding a few more elements internally to make a 300mm lens 400mm and adding a TC. The ratio of diffracted light to un-diffracted light remains the same at the same f-stop. Confusion may ensue because when you put the TC on the 300mm lens, what was before ƒ5.6 will now read as ƒ8. But you are using less of the frame. So you have kept the ratio of diffracted light the same. You will have to set the 300mm lens to ƒ4, for it to read ƒ5.6 on your meter. But on your top screen or viewfinder that ƒ4 will register as ƒ5.6 because the camera has done the conversion for you.

So in essence, both proposed answers are right, or both proposed answers are wrong depending on whether you're talking setting the aperture on a manual system or setting the exposure on an automated system

On the manual system with a 1.4 TC you have to manually set the aperture to ƒ4 to get ƒ5.6.
On an automated system, the camera sets the lens to ƒ4 (internally) for you when you dial in ƒ5.6

So on my K-1 (or on any auto aperture camera) with the Tammy 300 and 1.4 TC, internally the camera is set to ƒ4 to achieve ƒ5.6 with the TC, the camera does the calculation for me and appropriately sets the lens to the right aperture for the lens with the TC.

On my A-400 and 1.4 TC operated manually, I'd have to set the aperture to ƒ4 to achieve ƒ5.6. (But because of what aperture is, ƒ5.6 will still be less diffraction limited.)
Functionally on an auto-aperture system just set the Aperture you want, the camera will look after it. ƒ5.6 is ƒ5.6.
On a manual aperture sytem, you set the lens to ƒ4 to achieve ƒ5.6 with the lens and TC.

Different answers for different scenarios.

And as mentioned above, it's not really important.
ƒ4, ƒ5.6, and ƒ8 can usually be selected on an automated systems without reference to sharpness, they are all pretty close. Desired depth of field and treatment of out of focus areas for smooth bokeh is the appropriate thing to be looking at with reference to those apertures. You won't see a lot of difference in sharpness.

Both APS_c and FF are diffraction limited after ƒ5.6 on most lenses, but on FF, the diffraction limit is closer to ƒ8 than ƒ5.6. So what system you are using also plays a part. Going to ƒ8 on FF costs you less in terms of reaching the diffraction limit than APS-c does, in theory, in practice, you won't see any difference.

In any case, aperture is aperture, with a proper calculation of f-stop, f5.6 provides the highest lw/ph for a given subject, if and only if the whole subject is in focus. If you need more DoF then there may bet more lw/ph on the subject and even ƒ11 may produce more subject resolution. It's a rare photograph where the sharpest technical sharpness measure on the subject is produced at the lenses sharpest test chart setting. That only works on flat surfaces.

The way aperture is calculated, fƒ5.6 is f5.6 regardless of whether a TC is used for modern cameras, where the aperture is calculated by the camera, not found on the lens barrel.

As explained above, #2 is the correct answer. ƒ5,6 is the sharpest setting, and to achieve that with a 1.4 TC, the manual lens must be set to ƒ4. Because the TC was added, ƒ4 is really ƒ5.6 and will be the sharpest setting on test chart. As Lowell points out, the important factor is the ratio of the the circumference of the aperture ring (it's the inside edge of the aperture ring that causes diffraction) , to the amount of light coming through the lens. That is best represented by the true f-stop.

Without diffraction, the sharpest setting of a lens would be it's smallest aperture opening, ƒ22 or whatever. A pinhole camera is limited in sharpness only by diffraction.

Another take away from this is that the best lenses are diffraction limited at less than ƒ5.6. Because diffraction is always present, a really sharp lens will become diffraction limited with a lower diffraction to light ratio. If a lens is so sharp, the line pairs are so thin the image will start to degrade on test charts at much smaller diffraction ratios. I believe the best lenses start to degrade after ƒ2.8 although ƒ4 is more common among really sharp lenses. The finer the line pairs, the more likely diffraction is to affect them.

Another reason for not using ƒ5.6 lenses with 1.4 TCs and not using ƒ4 lenses with 2x TCs. The lens wide open already is past the diffraction limit, although shooting with a combination that wide open is a true ƒ8, that will be the least of your worries.

Last edited by normhead; 02-28-2019 at 10:55 AM.
03-01-2019, 12:45 PM   #14
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Simple approach- test under controlled conditions so you know what the various combos of your equipment are capable of. Then when you bork something during field use, you'd also have a better chance of figuring out what went wrong.

QuoteOriginally posted by Alex645 Quote
Counterpoint: Yes, the effective amount of light may become the equivalent of f/16, but the aperture is still f/8 on the lens. The TC does not convert the diffraction of the lens to what that lens would diffract at f/16
This is the opposite of what I've always understood. The effective aperture is what will determine the amount of diffraction, so the diffraction of your lens set to f/8 (via the aperture ring) with a 2xTC behind it will have the same diffraction as the lens on its own set to f/16.

Old post by falconeye: Do teleconverters count toward diffraction equation? - PentaxForums.com

I'd be interested in seeing anything you have to the contrary.

QuoteOriginally posted by Lowell Goudge Quote
i think one needs to consider diffraction quite differently, Diffraction is always present, even wide open, diffraction is the bending of light around an edge, in this case the aperture. The issue with diffraction is that as you stop down the lens, the amount of light that is projected to the image which is distorted by difraction, increases as a percentage of the total light emitted from the lens, diffraction is much worse, on shorter lenses because the aperture is relatively small to begin with, and therefore plays a much more important role than on a long lens.
This is also the opposite of what I've understood. Though perhaps calrrification is needed. Are you saying diffraction on a 20mm lens set to f/16 would be worse than the diffraction of a 200mm lens set to f/16? I have understood diffraction to be the same on a 20mm lens and a 200mm if both were set to the same f/16 aperture.

Have a look at this Cambridge in Colour article, jump to the "Technical Note" at the bottom Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

I would also be interested in seeing anything you have that contradicts the Cambridge in Colour note.
03-01-2019, 01:42 PM   #15
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QuoteOriginally posted by BrianR Quote
Have a look at this Cambridge in Colour article, jump to the "Technical Note" at the bottom Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks
I was hoping this could be resolved without going that far, it was a pretty simple question... but for those who want a deeper understanding it's good stuff.

Interesting topic for a poll.
Can you use polls to determine the correct answers to technical questions?
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