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SLR Magic 25mm -light gathering capabilities

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frnkoch

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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:
www.dxomark.com

News

www.dxomark.com www.dxomark.com

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??
 
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!
 
frnkoch said:
frnkoch wrote:

[...]
Click to expand...
frnkoch said:
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
Click to expand...
frnkoch said:
[...]
Click to expand...
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?
 
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.
 
frnkoch said:
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.
Click to expand...
Best way to test DOF is to photograph a ruler.
 
secretworld said:
secretworld wrote:

Best way to test DOF is to photograph a ruler.
Click to expand...
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.
 
frnkoch said:
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??
Click to expand...
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!
 
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.
 
coffeebean said:
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.
Click to expand...
I did use spot metering. Sorry, but after T1.2 the sensor just isn't able to use the additional light.
 
How about when it's completely dark outside? I'm curious to see how it works then.
 
honeyiscool said:
honeyiscool wrote:

How about when it's completely dark outside? I'm curious to see how it works then.
Click to expand...
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.
 
frnkoch said:
frnkoch wrote:
frnkoch said:
secretworld wrote:

Best way to test DOF is to photograph a ruler.
Click to expand...
I just did a ruler test, the lens behaves like T/1.2 when set to T/0.95, regarding depth of field.
Click to expand...
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...
 
If this is generally the case I'll have to rethink my plan to buy the 17.5 nokton
 
frnkoch said:
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??
Click to expand...
Arent you confusing F stops and T stops here. Your post doesnt really make sense to me?
 
Abrak said:
Abrak wrote:
Abrak said:
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??
Click to expand...
Arent you confusing F stops and T stops here. Your post doesnt really make sense to me?
Click to expand...
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.
 
robonrome said:
robonrome wrote:

If this is generally the case I'll have to rethink my plan to buy the 17.5 nokton
Click to expand...
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.
 
frnkoch said:
frnkoch wrote:
frnkoch said:
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.
Click to expand...
Click to expand...
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.
 
Abrak said:
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.
Click to expand...
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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