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Forum: Pentax SLR Lens Discussion
12-14-2017, 01:34 PM
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OK here is what I think happens. The fact that something up against the front element vignettes rather than dims an image is due to the two factors:
The relative tiny size of the exit element (8mm) compared to the front element. (46mm)
And the relatively long physical length of the lens – about 50mm compared to about 35mm for a 50 1.7
This relatively confining exit combined with the length makes for a fairly strong parallax effect which is just about non existent for say a 50 1.7. So one side of the sensor is looking at the other side of the front element rather than all it. This (like a pinhole camera) means there is some degree of focus on the surface of the front element.
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Forum: Pentax SLR Lens Discussion
12-14-2017, 11:11 AM
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Yes indeed I did that and you have it about right. And that is why I specified I was using the Original Q although I suspect it would still cover the later sensor. Of course the disc should in theory be invisible with just the image stopped down but it clearly doesn't work that way here just as it doesn't with a 28mm. Is there a simple explanation for that. Without checking further the disc is invisible on my 50mms and Takbayonet 135 as you would expect. I always thought with the 28s it was a function of retrofocus design.
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Forum: Pentax SLR Lens Discussion
12-13-2017, 09:57 PM
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OK I did this very quickly using lenscaps from an 18mm and a 24mm 110 lens as black discs. They are about 32mm and 26mm in diameter. The front element of a 110 70mm is about 46mm in diameter.
So;
Front element - 46mm – 1660 sqmm of glass
32mm diameter black disc – 800 sqmm (so half the light - 1stop)
26mm diameter black disc – 530 sqmm (2/3 the light – sort of ½ stop)
Original Q on AV no disc 1/320 sec
With 26mm – 1/15 sec - about 4.5 stops reduction.
With 32mm disc – no image
The disc sitting on the lens surface is not fully optically invisible.
The smaller disc can be seen as a small blob fully obscuring the centre of the image.
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Forum: Pentax SLR Lens Discussion
12-13-2017, 07:15 PM
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Yes I might have to fire up my Q batteries and try that.
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Forum: Pentax SLR Lens Discussion
12-13-2017, 04:09 PM
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The point was with the 110mm 70mm was that reducing the front element a stop utilizing a washer made little or no difference to the sharpness nor the speed of the lens. In short the lens did not seem to be utilising the outer area of the front element. Of course this was on the Q - I think 110 film is considerably larger so maybe there could be some vignetting. But the washer was hard down on the glass so should be close to optically invisible shouldn't it?
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Forum: Pentax SLR Lens Discussion
12-13-2017, 04:00 PM
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Sorry but I had only noticed it on the Nikon - I think it was when I had noticed some discussion about an earlier variety being a bit shy of 200mm or something . Your other examples are equally or more valid. You may understand now why I asked that original question to cyberjunkies post querying whether the front element had to be at least FL/f . It is pretty clear now everyone accepts that.
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Forum: Pentax SLR Lens Discussion
12-13-2017, 01:37 PM
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Yeah that is another way of what I mean the front element has to be at least FL/f. Regarding that I have always thought the Niknon 70 - 200 2.8 was cutting it fine - a 77mm filter thread and requiring 71.4mm to do 200mm/f2.8. Leaves 5.5mm (2.75mm) for edging.
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Forum: Pentax SLR Lens Discussion
12-10-2017, 11:39 PM
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Forum: Pentax SLR Lens Discussion
12-10-2017, 11:21 PM
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Can you clarify one thing? From common sense I have always presumed the front glass had to measure at least the Focal length divided by fastest aperture. Does that still hold true with what you say?
I agree that often it is larger presumably from design. I was experimenting with a Pentax 110 70mm and found I could put at least one stop of disc onto the surface of the front element and it did not alter the speed of the lens so I suspect it's front element was partly cosmetic.
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