[Nick Siebers]

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On my F50/1.4 there is a small amount of vignetting with the 49 --> 28mm step down ring wide open and close focused. This may or may not matter in actual practice. I used the same ring on a DA 70 in place of its hood for an amazingly compact package. And a 49-->37, 37-->30.5 double ring set-up on an FA 43, which does not obviously vignette.

[Mareket]

I read that in early days, apertures were placed in front of lenses. They do not vignette, they act to stop down.
As well as acting as a (probably more efficient) hood, for a 50 mm lens with a 28 mm hole in the adaptor ring, the lens has a new f/ number of 50/28 = f/1.8
I will try this on a f/2 50 mm

I've thought about that too, but I always get confused. Is the max aperture related to the smallest element or the smallest point the light passes through? Because surely the big 300/2.8 lenses (nevermind Sigma's monster) and even Pentax's 560/5.6 would need a lens mount 10 cm wide for it to be related to the thinnest point the light passes through.

Perhaps it's related to the largest refraction that can take place from the optical axis? Anyone know more?

And for the record I can't see any difference in the photos posted, though it is only half a stop...

[kh1234567890]

I've thought about that too, but I always get confused. Is the max aperture related to the smallest element or the smallest point the light passes through? Because surely the big 300/2.8 lenses (nevermind Sigma's monster) and even Pentax's 560/5.6 would need a lens mount 10 cm wide for it to be related to the thinnest point the light passes through.

Perhaps it's related to the largest refraction that can take place from the optical axis? Anyone know more?

And for the record I can't see any difference in the photos posted, though it is only half a stop...

You are not changing the entrance pupil size - the image of the physical aperture as seen from the front of the lens - which is what determines the f-number. In practice with the M 50/1.4 there is no difference to metering or depth of field with or without this hood. Just that the lens actually becomes useable at f1.4.

I could not resist taking it out for a spin late this afternoon - click on my sig, all taken at f1.4.

[aleonx3]

I use a lens hood all the time even on the M/K and M42 lens. Most of the M/K lenses have filter sizes of 49, 52, 55 and 58. I use 49 lens hood, 55 and 58 predominately with step-up rings. However, I never thought about using a step down ring as lens hood.

Using the appropriate hood, I noticed also the slight improvement of CA and flare control, but not as dramatic improvement as what you have shown here. So, perhaps, the step-down ring is a more effective lens hood than the conventional type.

[wombat2go]

The entrance pupil of a f/1.4 50 mm is 35.7 mm
According to Brandt -The Photographic Lens " For lenses with a front stop ( like the old landscape lenses) the stop diameter is also the effective aperture of the system. It determines the cross-section of the light beam passing through the lens. If the stop is behind the lens, it can no longer be the effective aperture for the lens element or lens system in front of the stop already converges or diverges the rays before they pass through the stop itself"
and " In most photographic lenses the stop is located within the optical system. In the simplest case it may be between a front and a rear element. Both produce an image of the stop which Abbe termed the entrance pupil. The image produced by the front lens component is the entrance pupil"

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

[Roob-N-Boots]

I can't find a 49mm to 28mm step down ring.... =(

49mm to 28mm Step Down Filter Ring Adapter 49 28 | eBay

[kh1234567890]

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

Whatever the cause or explanation may be, I'm happy - the annoying fuzzy flare at f1.4 has gone. There would appear to be some life in the old dog yet ..

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Last edited by kh1234567890; 03-21-2013 at 01:08 PM.

[kh1234567890]

49mm to 28mm Step Down Filter Ring Adapter 49 28 | eBay

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- nicna-store'

[Jonathan Mac]

These are incredible results. I hadn't realised that any kind of hood could make such a dramatic difference to sharpness. I always use a hood but for contrast and flare, although yes, I am aware that the drop in sharpness is due to flare, though not the type that we mostly worry about.

The hoods I use are the rubber ones, though usually the less common and longer three-stage ones. I haven't done any tests of sharpness with and without, though I might have to now...

[EarlVonTapia]

Wow, quite a difference!
Do you guys think it would be useful for my F 50mm 1.7, considering the front element is a bit recessed already, and i'm using a lens hood for 50mm lenses in fullframe?

I think the F 50 1.7 might be a rare case where a lens hood does not provide a significant increase in image quality, either by increasing contrast or reducing incoming light from the side. The front element is simply so recessed into the body. I did do some amateur tests with my copy when I first got it and could not see any discernible difference with my plain eyes (pixel peeping might tell a different story) with and without the hood, so I kept it off to keep the lens small and pocketable. It probably won't hurt though unless the hood is so narrow as to cause vignetting.

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.

[sTi]

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.

Or could it be that the coating of the FA is so much improved already compared to the K/M variant that a hood makes much less difference?

[tromboads]

Just stop down your 1.4 to f1.8(ish) and you will see the difference.

I to was so excited by this magical hood, then I thought about what just happened :P

[VaSA]

On the plus side though, those Chinese sellers are probably making record sales of step-down rings right now

[tromboads]

they are sitting there thinking to themselves. .. .. "those westerners be crazy "

[MSL]

These are incredible results. I hadn't realised that any kind of hood could make such a dramatic difference to sharpness. I always use a hood but for contrast and flare, although yes, I am aware that the drop in sharpness is due to flare, though not the type that we mostly worry about.

The hoods I use are the rubber ones, though usually the less common and longer three-stage ones. I haven't done any tests of sharpness with and without, though I might have to now...

+1 on that, although I haven't been using a hood much because I've mostly done close focus or indoor shooting. But I have to wonder how many of my lenses have left me underawed just because I haven't been using a hood.