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04-12-2015, 05:33 AM   #1
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Macro lens with no flat rear element

Hello guys
I have a 50mm macro lens (Pentax-FA 2.8 50 mm Macro) that gives me trouble: I get a blueish spot when I use F. 25-32. I've been told that it's cause by feflections from ar flat rear elemnet.
Look at: https://www.flickr.com/photos/bladt/16900869850/
I had the same problem with an old Tamron SP 2.5 90mm lens.
Can anyone tell me, if there's a 50mm Pentax Macro lens, that doesn't suffer from this problem, please?

I have the Pentax-F 2.8 100mm Macro, which doesn't have this problem and otherwise is excellent. I wish I had a 50mm like that :-)

Regards
Jens

04-12-2015, 06:44 AM   #2
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The K and M 50/4 Macro and the A50/2.8 macro all have curved rear elements and so shouldn't have this issue (I too experianced it with the Tamron). Intersting that the FA50/2.8 has this issue; it's element isn't flat, though it is not far from it.
04-12-2015, 07:41 AM   #3
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I thought F, FA, and DFA 50mm f2.8 macro all have the same basic optical design, 8 elements in 7 groups with 8 aperture blades, 1:1 magnification.
The main problem is that A and older do not go to 1:1 magnification without help.
Anyway, can't help you further, other than suggesting you go to a store and try a DFA 50mm (newer coatings, made for digital, might be better) and maybe third party macros in that range. Sigma 70mm macro is very highly regarded, but rare. Tamron never released its 60mm f2 for Pentax.. oh, there is that new K-mount Venus macro lens, with no automation, but very high magnification (2:1)
04-12-2015, 08:14 AM   #4
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Just out of curiosity, have you considered using a smaller format sensor instead of those really tiny apertures? You're losing sharpness beyond f/8-f/11 on APS-C anyway.

Maybe the Q shooters can chime in? To me that seems like a good application for them.

PS - nice photos!

04-12-2015, 08:30 AM   #5
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QuoteOriginally posted by Na Horuk Quote
I thought F, FA, and DFA 50mm f2.8 macro all have the same basic optical design, 8 elements in 7 groups with 8 aperture blades, 1:1 magnification.
The main problem is that A and older do not go to 1:1 magnification without help.
The D-FA, F and FA do share the same basic optical design (though the D-FA may be slightly tweaked in a couple of the elements), though as you say the new coatings would presumably help. The A has an entirely different design (6/4). As you say, it, and the earlier 50/4 only go to 1:2 natively, but one can always use tubes. There are a few third-party 50mm macro's as well: Sigma has one (f2.8), but I don't recall the shape of its rear element (latest version would be for digital and so should have coatings to minimise sensor reflection, though), Vivitar has an older 55/2.8, etc.

Focus stacking is the common way to get lots of DOF in macro shots, but the OP may not want to do that for the added complexity it brings.
04-12-2015, 09:31 AM   #6
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Interesting. It's hard to be certain that the flare spot is caused by a flat rear element without trying other lenses. Is there any chance it could be some other surface causing the reflection? I get something similar with an adapter I made for K to Mamiya mount, and also with an M42 adapter. In both cases I get a flare spot in the centre which becomes more pronounced on stopping down.

The only things I can think to try, beyond changing lenses are:

1) make a rectangular hood which eliminates all light from outside the frame

2) Use f22 or wider and focus stack to get the DOF you need - you should be able to do it with a couple of shots, which isn't too difficult to combine in Photoshop
04-12-2015, 02:30 PM   #7
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Well, for product photos the older lenses that only go to 1:2 magnification might be okay, as you don't usually need true 1:1 magnification for that type of photography, unless you have really small objects.
One other thing you can do is to calculate the DoF and use the widest aperture that lets you get the whole product within the DoF, but sometimes going above f16 is unavoidable

04-12-2015, 05:43 PM   #8
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I think it is unclear that it is the rearmost lens that has the reflection. Not macro/not digital but I recall one gets a flare spot with film with certain lens and TC combinations and (w/ same TC) not others.
04-17-2015, 12:29 AM   #9
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QuoteOriginally posted by macTak Quote
The K and M 50/4 Macro and the A50/2.8 macro all have curved rear elements and so shouldn't have this issue (I too experianced it with the Tamron). Intersting that the FA50/2.8 has this issue; it's element isn't flat, though it is not far from it.
Thanks a lot guys
With the FA-50mm I only have the problem at the three stops f. 25-29-32. But sometimes I need to use these stops, since a tremendous ammount of light is necassary/desirable.
I normally use focus stacking for jewelry and such. But I have to use more shots at F11 than F32 :-) More shots represents a problem with 50mm, since the sort of wide angle image will get distorted too much when movig the lens forward (w. a focus rail). So if I take too many shots for stacking, Photoshop will have problems blending the layers properly. It's much easier with a 100mm :-) But sometimes - for larger objects - the 100mm gets too close. So - sharpness is not an issue, as I use stacking - my images are very sharp. I just wish I didn't get the blue spot.

Full frame is not really an issue yet, since the new FF Pentax haven't arrived yet..

I may try to use the 645D for which I have a tremendously good 120mm macro lens :-)

Regards
Jens
04-17-2015, 01:41 AM   #10
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I just did a quick test with the SMC Pentax-FA 645 120mm F4 Macro.
No blue spot issue - this is a pro lens !
https://www.flickr.com/photos/bladt/16989355879/

Regards
Jens
04-17-2015, 04:23 AM   #11
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You need to clean your legos, Jens
04-17-2015, 07:25 AM   #12
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QuoteOriginally posted by Photodana Quote
With the FA-50mm I only have the problem at the three stops f. 25-29-32. But sometimes I need to use these stops, since a tremendous ammount of light is necassary/desirable.
At these f-stops loss of resolution due to diffraction will be huge You are talking about roughly f/128 at m=1, or about 1/4 the usual linear resolution--or 1/16 if one thinks of pixels--so 16 MP becomes 1 MP.

Also this suggests the reflection is from the aperture blades and not the rear element--as actually is often the case.

---------- Post added 04-17-15 at 08:24 AM ----------

Let me correct what I said about the loss of resolution. Likely closer to 1/2 linear resolution or effectively 16 --> 4 MP.

Reasoning as follows. 2 stops for m=1, likely another stop because this lens is not very symmetrical, so effectively 3 stops (f/16 --> about f/100). And with cropped sensor it is likely diffraction limited at about f/11. Thus 4 stops smaller (than f/11)--or 1/2 the resolution.

Last edited by dms; 04-17-2015 at 08:25 AM.
04-17-2015, 02:03 PM   #13
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I know, Jin Desu :-)
I was just testing the lens...
04-19-2015, 04:04 AM   #14
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You might also consider an enlarging lens (there are plenty of fine 50mm ones, and there are also many excellent 80mm ones, as well as some 60mm and 75mm ones); those seem tailor-made for your sort of purpose and can be had very inexpensively.
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