Barry: ASA Rating?

[Steve Mullen]

I agree -- I think 1.5 is the best you get. Just like when you say a 1.4 lens you mean the best.

The difference is that when you say a 1.4 lens we all know you mean the number f1.4 marked on the lens AND with a full Open aperture.

I think Fujinon is saying, yes it has a T-stop of 1.5 -- but not disclosing that this value is only reached at some-point in the aperture range. Maybe in the middle or maybe only around f/11. And, certainly not at full open!

Worse, even this 1.5 value may be only wide open. It could increase dramatically at full Tele.

Which means at full Tele and at f/1.4 the actual T-stop could be 5.6 or worse!

So to REALLY answer Charles's question -- someone needs to create a 2D table with Zoom across the top and f-stop down the side. Then put the ASA values in the cells. Also choose the cell with the highest ASA value and make it a 1. Then rate all the other cells in LOWERED SENSITIVITY is "stops of light."

[Tim Dashwood]

Originally Posted by Steve Mullen

I think Fujinon is saying, yes it has a T-stop of 1.5 -- but not disclosing that this value is only reached at some-point in the aperture range. Maybe in the middle or maybe only around f/11. And, certainly not at full open!

Huh????

Originally Posted by Steve Mullen

Worse, even this 1.5 value may be only wide open. It could increase dramatically at full Tele.

Which means at full Tele and at f/1.4 the actual T-stop could be 5.6 or worse!

4 stops? I highly doubt it. I just measured complete consistency from 5.5mm to 40mm and then a drop of 10 to 15 IRE with heavy vignetting as I approach 88mm. BTW, the vignetting seems to start subtley at around 20mm.

Originally Posted by Steve Mullen

So to REALLY answer Charles's question -- someone needs to create a 2D table with Zoom across the top and f-stop down the side. Then put the ASA values in the cells. Also choose the cell with the highest ASA value and make it a 1. Then rate all the other cells in LOWERED SENSITIVITY is "stops of light."

A simpler and better solution would to simply measure actual transmittance at each marked F stop using the published spec of T1.5 as the starting point for full open. However, based on Barry's experience with his lens, each lens would probably have slight variances with results.

[Charles Papert]

Yes indeed. I wasn't looking at the variance in light transmittal across the zoom range, only the question of why the sensitivity of the camera appeared to shift across the APERTURE range, so my theory and suggested experiment were designed to measure the actual transmittance (i.e. T-stops) of the lens against the markings (f-stops) which I thought might shed light on the conundrum. I can't see why a camera would exhibit different sensitivity (ASA) at different apertures--the imager doesn't "care" what the aperture setting is, it only knows the amount of light hitting it.