SLR Magic 25mm -light gathering capabilities

Started Aug 14, 2013 | Discussions
frnkoch New Member • Posts: 14
SLR Magic 25mm -light gathering capabilities

Hi Folks

A few days ago I received an SLR Magic 25mm T0.95 lens. I immediately began doing some initial tests, especially of the light gathering abilities when shooting wide open. To be honest, I was shocked.

At apertures above T1.4 the lens behaves perfectly normal, with each aperture stop corresponding to one EV more light gathered. This is how a lens should behave. But beyond T1.4 and up to T0.95, the lens does not gather much more light. In fact, when turning the aperture from T1.4 to T0.95, it only gathers about 0.3 light units more light, when it should be 1EV in theory.

I did some research and it turns out that this seems to be a problem of modern sensors, not of the lens itself. At very small apertures (F/1.2 and smaller) the sensors fail to use the additionally available light. Here is an article explaining the effect:
http://www.dxomark.com/index.php/Publications/DxOMark-Insights/F-stop-blues
It turns out that for low light shooting, a very large aperture is almost useless. You gain very little by going from F/1.2 to F/0.95, except a thinner depth of field.
What do you guys think? Should I just send the lens back??

secretworld Senior Member • Posts: 1,734
Re: SLR Magic 25mm -light gathering capabilities

I don´t think you will get a much thinner DOF either. Since the light from the widest part of the aperture is hardly seen by the sensor. But if you like the lens at 1.2 or how it renders at 0.95 keep it!

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Pedagydusz Veteran Member • Posts: 5,638
Re: SLR Magic 25mm -light gathering capabilities

frnkoch wrote:

[...]

I did some research and it turns out that this seems to be a problem of modern sensors, not of the lens itself. At very small apertures (F/1.2 and smaller) the sensors fail to use the additionally available light. Here is an article explaining the effect:

http://www.dxomark.com/index.php/Publications/DxOMark-Insights/F-stop-blues

[...]

It also depends on the camera (sensor) itself, apparently (according to that article and others) some cameras are more sensitive to that issue than others.

Which camera were you using?

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OP frnkoch New Member • Posts: 14
Re: SLR Magic 25mm -light gathering capabilities

Indeed, the depth of field doesn't change much past T/1.4 either, just tested it. Although it is hard to quantify this aspect, I might be wrong here. 
But it is quite a disappointment, considering that the lens is VERY expensive and relatively hard to use.

OP frnkoch New Member • Posts: 14
Re: SLR Magic 25mm -light gathering capabilities

Pedagydusz wrote:

It also depends on the camera (sensor) itself, apparently (according to that article and others) some cameras are more sensitive to that issue than others.

Which camera were you using?

I tested it on an Olympus OMD.

secretworld Senior Member • Posts: 1,734
Re: SLR Magic 25mm -light gathering capabilities

frnkoch wrote:

Indeed, the depth of field doesn't change much past T/1.4 either, just tested it. Although it is hard to quantify this aspect, I might be wrong here.
But it is quite a disappointment, considering that the lens is VERY expensive and relatively hard to use.

Best way to test DOF is to photograph a ruler.

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OP frnkoch New Member • Posts: 14
Re: SLR Magic 25mm -light gathering capabilities

secretworld wrote:

Best way to test DOF is to photograph a ruler.

I just did a ruler test, the lens behaves like T/1.2 when set to T/0.95, regarding depth of field.
I do have some good news to report, though. Stopped down to T/2.0 the lens becomes extremely sharp! It easily beats my oly 17mm f/1.8 in centre sharpness. I will test the corners next.

tjuster1 Senior Member • Posts: 2,222
Re: SLR Magic 25mm -light gathering capabilities

frnkoch wrote:

Hi Folks

A few days ago I received an SLR Magic 25mm T0.95 lens. I immediately began doing some initial tests, especially of the light gathering abilities when shooting wide open. To be honest, I was shocked.

At apertures above T1.4 the lens behaves perfectly normal, with each aperture stop corresponding to one EV more light gathered. This is how a lens should behave. But beyond T1.4 and up to T0.95, the lens does not gather much more light. In fact, when turning the aperture from T1.4 to T0.95, it only gathers about 0.3 light units more light, when it should be 1EV in theory.

I did some research and it turns out that this seems to be a problem of modern sensors, not of the lens itself. At very small apertures (F/1.2 and smaller) the sensors fail to use the additionally available light. Here is an article explaining the effect:
http://www.dxomark.com/index.php/Publications/DxOMark-Insights/F-stop-blues
It turns out that for low light shooting, a very large aperture is almost useless. You gain very little by going from F/1.2 to F/0.95, except a thinner depth of field.
What do you guys think? Should I just send the lens back??

Wow, really interesting. Even more concerning is the inverse correlation between pixel pitch and light-loss--modern m43 cameras, with their tiny pixel pitch (lower than anything in that diagram), would be expected to experience the most light loss.

Also makes me wonder if this is an argument against the trend toward more and more megapixels, which we're once again seeing. The new Sony NEX is rumored to have 32 MPix for example!

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coffeebean Regular Member • Posts: 102
Re: SLR Magic 25mm -light gathering capabilities

If you are using average metering, whatever light you gain the central portion gets offset by the lightloss in the border region from vignetting. Spot metering will give you a better idea.

OP frnkoch New Member • Posts: 14
Re: SLR Magic 25mm -light gathering capabilities

coffeebean wrote:

If you are using average metering, whatever light you gain the central portion gets offset by the lightloss in the border region from vignetting. Spot metering will give you a better idea.

I did use spot metering. Sorry, but after T1.2 the sensor just isn't able to use the additional light.

honeyiscool
honeyiscool Senior Member • Posts: 1,376
Re: SLR Magic 25mm -light gathering capabilities

How about when it's completely dark outside? I'm curious to see how it works then.

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OP frnkoch New Member • Posts: 14
Re: SLR Magic 25mm -light gathering capabilities

honeyiscool wrote:

How about when it's completely dark outside? I'm curious to see how it works then.

Same thing, really. I did some shooting last night. Going from T/1.4 to T/0.95 gives about 0.3 EV.
My conclusion: Don't bother with super fast apertures.

Chez Wimpy
Chez Wimpy Veteran Member • Posts: 9,055
Re: SLR Magic 25mm -light gathering capabilities

frnkoch wrote:

secretworld wrote:

Best way to test DOF is to photograph a ruler.

I just did a ruler test, the lens behaves like T/1.2 when set to T/0.95, regarding depth of field.

Something to keep in mind regarding the upcoming (f0.95 and f1.2) 42.5's...

FF suffers from this problem as well, but in that case, there aren't many (faster than f1.4) lenses to worry about.  Ironic that f0.95 on m43 emerged as a way to match fast primes on FF for DOF control, yet...

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-CW

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SeeRoy Senior Member • Posts: 1,666
Re: SLR Magic 25mm -light gathering capabilities
2

But, but, but... what's the "unboxing experience" like?

Roy

robonrome
robonrome Senior Member • Posts: 2,334
Re: SLR Magic 25mm -light gathering capabilities

If this is generally the case I'll have to rethink my plan to buy the 17.5 nokton

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Abrak Veteran Member • Posts: 4,186
Re: SLR Magic 25mm -light gathering capabilities

frnkoch wrote:

Hi Folks

A few days ago I received an SLR Magic 25mm T0.95 lens. I immediately began doing some initial tests, especially of the light gathering abilities when shooting wide open. To be honest, I was shocked.

At apertures above T1.4 the lens behaves perfectly normal, with each aperture stop corresponding to one EV more light gathered. This is how a lens should behave. But beyond T1.4 and up to T0.95, the lens does not gather much more light. In fact, when turning the aperture from T1.4 to T0.95, it only gathers about 0.3 light units more light, when it should be 1EV in theory.

I did some research and it turns out that this seems to be a problem of modern sensors, not of the lens itself. At very small apertures (F/1.2 and smaller) the sensors fail to use the additionally available light. Here is an article explaining the effect:
http://www.dxomark.com/index.php/Publications/DxOMark-Insights/F-stop-blues
It turns out that for low light shooting, a very large aperture is almost useless. You gain very little by going from F/1.2 to F/0.95, except a thinner depth of field.
What do you guys think? Should I just send the lens back??

Arent you confusing F stops and T stops here. Your post doesnt really make sense to me?

OP frnkoch New Member • Posts: 14
Re: SLR Magic 25mm -light gathering capabilities

Abrak wrote:

frnkoch wrote:

Hi Folks

A few days ago I received an SLR Magic 25mm T0.95 lens. I immediately began doing some initial tests, especially of the light gathering abilities when shooting wide open. To be honest, I was shocked.

At apertures above T1.4 the lens behaves perfectly normal, with each aperture stop corresponding to one EV more light gathered. This is how a lens should behave. But beyond T1.4 and up to T0.95, the lens does not gather much more light. In fact, when turning the aperture from T1.4 to T0.95, it only gathers about 0.3 light units more light, when it should be 1EV in theory.

I did some research and it turns out that this seems to be a problem of modern sensors, not of the lens itself. At very small apertures (F/1.2 and smaller) the sensors fail to use the additionally available light. Here is an article explaining the effect:
http://www.dxomark.com/index.php/Publications/DxOMark-Insights/F-stop-blues
It turns out that for low light shooting, a very large aperture is almost useless. You gain very little by going from F/1.2 to F/0.95, except a thinner depth of field.
What do you guys think? Should I just send the lens back??

Arent you confusing F stops and T stops here. Your post doesnt really make sense to me?

Here is what confused you: The article by dxomark uses F stops, my lens has T stops. Thus I spoke of T stops when describing my measurements and of F stops when summarizing the article.

OP frnkoch New Member • Posts: 14
Re: SLR Magic 25mm -light gathering capabilities

robonrome wrote:

If this is generally the case I'll have to rethink my plan to buy the 17.5 nokton

Well, I'm having a bout of buyers remorse now. Don't get me wrong, you still get something like a T1.2 or T1.1, which is already pretty amazing. But it doesn't seem to be worth the high price.

Abrak Veteran Member • Posts: 4,186
Re: SLR Magic 25mm -light gathering capabilities

frnkoch wrote:

Here is what confused you: The article by dxomark uses F stops, my lens has T stops. Thus I spoke of T stops when describing my measurements and of F stops when summarizing the article.

No you are the one I think is confused. Your lens is measured in T stops - it is a T0.95. A lens like the the PL 25 1.4 is measured in F stops. By definition T1.4 will gather more light than F1.4. By definition an F0.95 lens will have less than 1 stop difference in light to an F1.4 stop.

A T stop is a measurement of light gathered by the lens - an F stop is not. If you are claiming that at T0.95 lens only gather 0.3EV additional light compared to T1.4, you are simply claiming that SLR Magic are fraudulent which I highly doubt and I dont think you meant to imply.

OP frnkoch New Member • Posts: 14
Re: SLR Magic 25mm -light gathering capabilities

A T stop is a measurement of light gathered by the lens - an F stop is not. If you are claiming that at T0.95 lens only gather 0.3EV additional light compared to T1.4, you are simply claiming that SLR Magic are fraudulent which I highly doubt and I dont think you meant to imply.

Not quite. The lens does gather an additional 1.0EV when going from T1.4 to T0.95. I trust SLR Magic. But, as stated in the dxomark article I have linked, the imaging sensor fails to use this additional light. This is due to the fact that a part of the light from very large aperture settings hits the sensor from an unfavourable angle, reducing the ability of the micro lenses to focus this light on the photo diode.

In fact, I am in contact with SLR Magic and they have given a partial confirmation that this problem does exist.

Conclusion: The SLR Magic 25mm is still the fastest M43 lens on the market. It is just not as fast in practice as in theory.

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