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03-31-2022, 09:02 AM   #1
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1.4x converter effect on minimal focus distance

I have a Pentax 1.4x converter on order and am aware of the reduction of light. But I haven't seen if (or how) it affects the minimal focus distance. I assume it doesn't affect minimal focus distance but thought I'd ask.


So the scenario I have in my head...
Getting further away from a subject will increase depth of field. So if I use the 1.4x on my 90mm macro, I can move back and increase depth of field (yes, opening the aperture for more light could cancel this). Or I could keep the same distance and have an even larger photo to work with assuming the minimal focus distance isn't affected.

03-31-2022, 09:27 AM - 2 Likes   #2
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You can get 1:1 with more working distance or 1:1.4 with the same working distance.

I use my 1.4xtc with the sigma 70. I also use it with my 55-300 to get butterflies filling the frame from 2 paces.
03-31-2022, 10:10 AM - 1 Like   #3
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By their design - the minimum focus distance will not change with a rear teleconverter. The depth of field also will increase since the maximum aperture will also get narrower by one stop. But at macro distances the reality is that the increase will be tiny. Even stopping down heavily has a limited effect (even if ignoring diffraction) on dof at these subject distances.
03-31-2022, 10:50 AM   #4
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QuoteOriginally posted by UncleVanya Quote
By their design - the minimum focus distance will not change with a rear teleconverter. The depth of field also will increase since the maximum aperture will also get narrower by one stop. But at macro distances the reality is that the increase will be tiny. Even stopping down heavily has a limited effect (even if ignoring diffraction) on dof at these subject distances.
I've noticed dof is pretty shallow even using f8-f11. The bigger benefit is the 90mm vs 126mm will give me a little more distance from my subject. Since I prefer spiders, insects, etc., the extra distance means the subject is less likely to get scared and run. Too many times I moved in an extra inch and the subject ran... If it helped with dof, that would be even better but I'm not too surprise that benefit is minimal.

03-31-2022, 11:54 AM - 1 Like   #5
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QuoteOriginally posted by jspi Quote
If it helped with dof, that would be even better but I'm not too surprise that benefit is minimal.
It doesn't halp. DoF is basically determined buy your magnification (critter : size of it on the sensor) and aperture.

Imagine that you go back to compensate for the longer focal length (same magnification), set the same aperture (the lens' aperture needs to be 1.4x wider), you have the same angle between the light rays contributing to the picture (from middle to corner), thus giving you the same DoF. This is a useful simplification, and there are of course subtle differences, but as a rule of thumb a TC used at the diffraction limit in macro shots for a constant magnification is 'neutral', it just gives higher working distance and a little bit of perspective change - often not very obvious in macro shots. However, it reaches higher magnification if you do not back up.
03-31-2022, 02:37 PM   #6
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QuoteOriginally posted by jspi Quote
Since I prefer spiders, insects, etc., the extra distance means the subject is less likely to get scared and run.
The extra working distance is more important in that your lens doesn't block the light source. I use f/19 with my ks-2 and 70mm 1.4x combo and still too little DOF.

at f/16 I have just a damselfly eye worth. I still had to crop too since they are so small.

04-01-2022, 01:24 PM   #7
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The minimal focus distance is not affected by the rear tele convertor, as stated above,
You can decrease the minimal focus distance with extension tubes (at the expense of loosing infinity focus).
And both techniques can be combined....

Unfortunately most extension tubes are contactless, turning your lens into a manual focus&aperture lens...

05-02-2022, 11:33 AM   #8
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QuoteOriginally posted by UncleVanya Quote
By their design - the minimum focus distance will not change with a rear teleconverter. The depth of field also will increase since the maximum aperture will also get narrower by one stop.
But at the same time the increase in focal length will reduce the dof...
05-02-2022, 11:37 AM   #9
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QuoteOriginally posted by mlag Quote
But at the same time the increase in focal length will reduce the dof...
True. I donít recall if the trade off is equivalent or not. Iíd guess a depth of field calculation is needed.
05-02-2022, 01:45 PM   #10
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Depth-of-field, focal length and aperture have a very fixed relationship.
To make for easy numbers, if the depth of field of a lens at f/4 is 1ft. at 10ft., it'll still be 1ft at 15ft. with a 1.5x adaptor, or 1ft at 20ft with a 2x adaptor, assuming the aperture setting isn't adjusted.
Obviously, there will be light loss due to the converter, so something will have to change to maintain exposure.
If the depth of field is to be maintained, either the ISO will need to be increased, the shutter speed decreased, or a combination of both.
It'll be a personal decision at exposure time to choose which option to take … give thanks for SR and pray for good light … a monopod or tripod might help
05-03-2022, 04:07 AM   #11
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QuoteOriginally posted by swanlefitte Quote
You can get 1:1 with more working distance or 1:1.4 with the same working distance.
I respectfully think you inverted the magnification ratio a converter provides on a macro lens. You should have stated "The magnification ratio then becomes "1,4 to 1" (larger) instead of "1 to 1,4" (smaller) at the closest focusing distance possible with the lens".

(1,4 divided by 1 = 1,4 X while 1 divided by 1,4 = 0,7 X)

Regards
05-03-2022, 07:36 AM   #12
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QuoteOriginally posted by RICHARD L. Quote
I respectfully think you inverted the magnification ratio a converter provides on a macro lens.
I did. You are right. It's good you point it out as it could definitely cause confusion. Thanks.
05-03-2022, 11:15 AM   #13
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Pentax-DA 1.4x AW AF Rear Converter Review - Macro Photography | PentaxForums.com Reviews
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