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03-21-2013, 06:31 PM   #31
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QuoteOriginally posted by EarlVonTapia Quote
Contrast this with the F 28 2.8 which has the exact same body as the F 50 1.7, but the front element protrudes like an insect eyeball. Even the tiniest bit of light from the side causes all kinds of weirdness. I've fixed that issue with some stacked filter and step-up rings to form a solid immovable hood.
I believe the F28 2.8 has the same forumulation as the A 28 2.8, and it is this lens which has most disappointed me, and which perhaps I should really give another test drive.

03-21-2013, 07:51 PM   #32
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I have seen lots of people have trouble with the A28/2.8. I never did, but I don't dismiss the ones that had trouble. The F coatings definitely look different, maybe they are better, I don't know.

I did almost all my Single in February shots with a hood about 1" deep. It doesn't vignette but the results here suggest going absolutely as far as you can.


03-21-2013, 09:59 PM   #33
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QuoteOriginally posted by MSL Quote
I believe the F28 2.8 has the same forumulation as the A 28 2.8, and it is this lens which has most disappointed me, and which perhaps I should really give another test drive.
I sold my F28 F/2.8 (attached to the camera in photo below) to a poster on this forum. I used it with a 49-28mm stepdown ring. As far as I know, the current owner is still using it that way.

03-22-2013, 12:31 AM   #34
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QuoteOriginally posted by wombat2go Quote
So for a f/1.4 50 mm I take it that any ring smaller than 35.7 mm will reduce the entrance pupil and hence increase the max f/
I put the SMC Pentax A 1:1.4 50mm on the k-01 and set f/1.4 and fixed iso 1600 and Av mode
The shutter speed was 1/800th
When a 28mm stop was placed over the nose of the lens, the shutter moved to 1/500th
Does it affect DoF?

03-22-2013, 03:51 AM - 1 Like   #35
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QuoteOriginally posted by sTi Quote
I'd like to see a comparison between f 1.4 with step-down and f 1.8 without step-down adapter to rule out that what we're seeing here is just the effect of stopping the lens down. I'm skeptical because my own tests with a FA 50/1.4 and a deep hood did not show any difference with or without the hood.
You are right.

All taken at the same exposure and same post-proc exposure tweak. Top row without hood f1.4, f2, f2.8. Bottom row with this hood f1.4, f2, f2.8.



Last edited by kh1234567890; 03-22-2013 at 04:01 AM.
03-22-2013, 04:14 AM   #36
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This it would seem is merely stopping down from 1/4 to 1/7 mechanically which is well known to improve lens sharpness, except now in this case you are no longer using a 1/4 lens. Not magic in my view.
03-22-2013, 04:42 AM   #37
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Well, if there is one advantage in using the stop down ring- is maintaining the circular bokeh. I use my F 50mm 1.7 at f2 almost all the time, because i noticed that at f2 the sharpness is better than wide open, but the bokeh starts to get a bit freaky at f2, when there are strong lights on the background.
03-22-2013, 06:26 AM   #38
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Thanks for the test, kh1234567890, confirming that the ring does restrict the entry pupil (and so the effective f-stop). I can't say I'm surprised--if you go back to your original comparison, the tree branches in the foreground look more OOF in the bare shot than in the stepping shot. I did once see a test here of a 50/1.4 with and without a deep hood, and it certainly did make a difference, although not as extreme. Now the question is whether its better to use a 49mm to 37mm stepdown ring as a hood, or just a deep cylindrical hood (e.g. the Takumar 135mm hood which I and others use). It still seems to me that the cylindrical hood will block more off-axis light than the stepdown ring, and further, most hoods have a matte interior, whereas the stepdown rings I have are all a rather reflective black surface.

Finally, wombat2go, can you give us the formula for calculating the entrance pupil?

03-22-2013, 06:59 AM   #39
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QuoteOriginally posted by macTak Quote
Finally, wombat2go, can you give us the formula for calculating the entrance pupil?
Hi Mac,
From my reading, I would answer that question as follows:
When a parallel beam of light from infinity arrives at a lens, the entrance pupil is the diameter that gets through the stop and focusses on the image plane.
That diameter is the diameter projected back through the rear nodal plane N2 and onto the front nodal plane N1. ( that is, it is measured at N1)
A real lens may have more complicated paths, and from the geometric optics diagrams it can be seen how the real diaphragm can be smaller than the entrance pupil,

However I think the following always stands, for light from infinity:

By definition, the entrance pupil is calculated by the f/ number the lens is set to
For example a 50 mm lens at f/2, the entrance pupil is 25 mm

A good reference for all this is Arthur Cox " Photographic Optics" and I see it is still available
03-22-2013, 08:42 AM   #40
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Here's a potential aid in comparing hood and aperture effectiveness.

PhotoMe offers this detailed graphic look at the matrix metering EV readings. Assuming that the lighting and camera position remain consistent, the cell readings from the matrix mode offer comparative insight into the light striking the sensor. The onset of vignetting is pretty obvious. Relative changes in the center cells MAY reveal off-axis dispersion/diffraction effects and overall hood effectiveness.

Paired screen captures are useful means for recording direct comparisons. Freestone Image Viewer offers a convenient Compare Mode with EXIF and histogram data displayed.

I've used this technique to determine the most effective length for custom hoods. Simply slide a black paper tube back and forth note the resulting changes. Of course this can also be done just by observing the changes in TTL metering, but this method is more definitive when using the tulip shaped hoods and can be saved for later review.

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03-22-2013, 08:50 AM   #41
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That's a great idea pacerr for an empirical way of comparison as opposed to the usual "eyeballed" comparisons.

Last edited by macTak; 03-22-2013 at 09:09 AM.
03-22-2013, 11:36 AM   #42
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QuoteOriginally posted by macTak Quote
That's a great idea pacerr for an empirical way of comparison as opposed to the usual "eyeballed" comparisons.
Too many uncontrolled variables to call it 'empirical' - more like a clue. But it's been a reasonably reliable method to support more subjective opinions and to record differences for later consideration. Marginal notes help.

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03-23-2013, 05:42 AM   #43
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If it affects the f-stop at large apertures without impacting the bokeh, the stop-down hood is still excellent in daylight, when the chromatic aberrations and the softness issue are worst.

Last edited by causey; 03-23-2013 at 08:41 AM.
03-28-2013, 07:26 AM   #44
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Some test shots with this hood on a Pentax-A 50mm f1.7 lens here.
04-03-2013, 07:06 AM   #45
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I've tried this with two hoods stacked on top of each other and the results are exactly the same as with only one normal rubber hood. I will keep my eye open for a stop-down ring to give it a go though.
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