Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
67gtonr Senior Member • Posts: 1,130
Re: Fast lenses, and High ISO

EinsteinsGhost wrote:

67gtonr wrote:

EinsteinsGhost wrote:

67gtonr wrote:

EinsteinsGhost wrote:

Fast lenses, as they are called allow for more light to hit the sensor and in turn allow for fast(er) shutter speeds. The F number provides a relative measure of how much this ability is.

In this age of ever increasing ISO, are fast lenses needed anymore? The only ability I see the fast lenses provide was actually a disadvantage that happened to become a feature, and that is shallow DoF, allowing for separation of subject from the background.

That said, should they be called Fast Lenses or Shallow Lenses

Now, I took some available light family portraits 750ft below ground at Carlsbad Caverns recently using Sony NEX-6. There isn't much light there (in this case, there was some light from the cafe but still too dark to see in person). I had Minolta 50/1.4 on Speedbooster, which gave me an effective 35mm f/1 lens. I used ISO 3200 and still had only 1/30s for shutter speed (could have improved it a bit with spot metering but some of the ambiance would be lost). The scene brightness value was -5EV. This is an example of when just a superfast lens or high ISO capability worked better when combined.

Would I have loved having Sony a7s instead? You bet! Would have gotten same exposure at ISO 6400 which is impressively clean and composed in that camera.

I am confused by this, I thought when a Speedbooster is used on an APS-C sensored camera it essentially negated the crop factor, so instead of your 50/1.4 shooting as a 75/1.8 it shoots 50/1.4, approximately?

It is not an exact compensation (IIRC, 0.71x, so almost). So, using 50/1.4 via speed booster is like using 35/1 without it (the resulting FOV and DOF will be that of about 50/1.5 on FF).

So the Speedbooster gave you a 50/1.5 not a 35/1, if you had used the same lens and Speedbooster on a full frame camera it would have given you 35/1.

No, the lens via SB works as if it is a 35/1 lens. But, if you put 35mm lens on APS-C, you get (approx) field of view of 50mm lens on FF. The DOF equivalence is f/1.5. However, exposure is determined for f/1.

My last reply was being typed as this was being posted. I was initially reading it that the booster was giving an image w/ a 35mmFOV and a F1 equivalent picture and that is not how I understood Speedboosters to work.

So does the auto exposure meter for the f1 correctly or do you adjust it manually?

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