a55 low light focus?

hi all,

so the 70/30 split of the transmitted/reflected light by the semi-translucent mirror only represents a loss of about 0.5 stops to the image sensor.

but what about the loss of light to the af module? that's about 1.7 stops, which is significant. how is the a55 at focusing in low light?

any opinions?

Light normally gets split to the OVF and the AF sensor anyway, Afaik. No Idea what the proportion is.

I dont have an opinion, though it's an interwsting point I hadnt considered or seen brought up....and it doesnt seem to affect AF normally...

But why is it that no DSLRs that i've seen have little red AF assist lamps on the flash popup like P&Ss? i've seen nikons with bigwhite AF assists but the little red AF assist is unobtrusive, costs no money (price of 1 red LED) and will allow low light focusing?

with the low light focusing ability of the A33 (same AF system as the A55). The only AF sensor that is worthy is the center sensor and it quits giving information in the amount of light I would think should be plenty sufficient.

These designs are in bad need of a dedicated AF assist light, the pop up flash isn't the best solution in many cases when you are trying to be discreet.

C

Jens_G said:

Light normally gets split to the OVF and the AF sensor anyway, Afaik. No Idea what the proportion is.

Click to expand...

good point. although the light will now have to be split between an evf and a af sensor. perhaps the evf takes less light than an ovf, but it'll have to be a very significant effect to counter for 1.7 stops...

gillbod said:

hi all,

so the 70/30 split of the transmitted/reflected light by the semi-translucent mirror only represents a loss of about 0.5 stops to the image sensor.

but what about the loss of light to the af module? that's about 1.7 stops, which is significant. how is the a55 at focusing in low light?

any opinions?

Click to expand...

If you look in your existing DSLR, you'll see a small secondary mirror behind the semi-silvered main mirror which draws some (But not all) light away for autofocus purpose.

I imagine that the two are comparable in terms of light gathering for AF.

edit:

gillbod said:

good point. although the light will now have to be split between an evf and a af sensor. perhaps the evf takes less light than an ovf, but it'll have to be a very significant effect to counter for 1.7 stops...

Click to expand...

The vast majority goes to the main image sensor, which then feeds ti the EVF. No light gets redirected to the EVF itself.

--
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Dan Vincent said:

gillbod said:

hi all,

so the 70/30 split of the transmitted/reflected light by the semi-translucent mirror only represents a loss of about 0.5 stops to the image sensor.

but what about the loss of light to the af module? that's about 1.7 stops, which is significant. how is the a55 at focusing in low light?

any opinions?

Click to expand...

If you look in your existing DSLR, you'll see a small secondary mirror behind the semi-silvered main mirror which draws some (But not all) light away for autofocus purpose.

I imagine that the two are comparable in terms of light gathering for AF.

edit:

Dan Vincent said:

good point. although the light will now have to be split between an evf and a af sensor. perhaps the evf takes less light than an ovf, but it'll have to be a very significant effect to counter for 1.7 stops...

Click to expand...

The vast majority goes to the main image sensor, which then feeds ti the EVF. No light gets redirected to the EVF itself.

Click to expand...

this clears things up for me. thanks!

gillbod said:

although the light will now have to be split between an evf and a af sensor.

Click to expand...

No, looks like you don't know what an EVF is and how it works

gillbod said:

hi all,

so the 70/30 split of the transmitted/reflected light by the semi-translucent mirror only represents a loss of about 0.5 stops to the image sensor.

but what about the loss of light to the af module? that's about 1.7 stops, which is significant. how is the a55 at focusing in low light?

any opinions?

Click to expand...

First as posted by others, a regular DSLR only direct a portion of the incoming light to the AF module aswell, second it's not so much the amount of light that is important for a phase detect AF module, but the angle of light.

daho said:

No, looks like you don't know what an EVF is and how it works

Click to expand...

that's a very condescending way of phrasing it. i do have an idea of how an evf works, and there are several ways an evf can be implemented. i was wrong about the a55, but no need for the condescending tone.

Are you speaking as an owner of an SLT camera? The users I've seen speak of the A33/A55 as having excellent low-light AF.

Statross2 said:

I dont have an opinion, though it's an interwsting point I hadnt considered or seen brought up....and it doesnt seem to affect AF normally...

But why is it that no DSLRs that i've seen have little red AF assist lamps on the flash popup like P&Ss? i've seen nikons with bigwhite AF assists but the little red AF assist is unobtrusive, costs no money (price of 1 red LED) and will allow low light focusing?

Click to expand...

So far, Sony has included the red assist light in the A700 and higher cameras. My first DSLR (Konica Minolta 7D) did not have the red light either (a significant shortcoming for a Minolta camera at that level) so I bought an old Minolta 2000xi flash for about $15.

This flash has the same Minolta/Sony flash shoe and is only a couple of inches tall. Its not suitable for use as a flash on the DSLRs (only fires at full power and is so close to the lens that you get red eye), but when it is turned off , the red AF illuminator on it will work when the camera needs help focusing. And its light weight and small enough.

I don't know if it will still work on a SLT camera, but I think it should.

tom

How can I have an opinion otherwise?

Maybe those owners have low expectations, the A33 does not focus well in low light.

Carl

cgarrard said:

How can I have an opinion otherwise?

Maybe those owners have low expectations, the A33 does not focus well in low light.

Carl
--

Click to expand...

In your opinion, Carl. Focus numerique said it was really good:

"In theory, the AF module of the new Alpha 55 and 33 should be good enough in low light, the sensors receiving a little more light (approximately one third on to the autofocus and 2 / 3 for the primary sensor). In fact, the autofocus of the Alpha 55 is particularly reactive. In low light, it reacts really well. The sheet shows a sensitivity range from -1 to 18 EV. When light comes down, the AF of the Alpha 55 is still fast. In this regard, progress is undeniable. "

http://translate.google.co.uk/translate?hl=en&sl=fr&tl=en&u=http%3A%2F%2Fwww.focus-numerique.com%2Fsony-alpha-33-55-deja-prise-mains-news-2091.html

Who to believe...

This shouldn't be opinion based anyway, low light AF performance is a quantifiable parameter. If Carl develops a rigorous and robust testing methodology and then applies this to 4-5 cameras similar to the A33 and shows us the results then I will accept whatever result he finds... Opinions, on the other hand, are worth nothing.

It's a very overcast afternoon here today and in my house, it is dull light, I have just tried the auto focus in the house at dark locations and I am getting a focus lock at F6.3 and shutter at 1/3 without the flash strobing etc.

0.8" & F4 still works,

but beyond that, I need the flash up to strobe.

Panasonic G1 series and such have red assist lamps

Statross2 said:

I dont have an opinion, though it's an interwsting point I hadnt considered or seen brought up....and it doesnt seem to affect AF normally...

But why is it that no DSLRs that i've seen have little red AF assist lamps on the flash popup like P&Ss? i've seen nikons with bigwhite AF assists but the little red AF assist is unobtrusive, costs no money (price of 1 red LED) and will allow low light focusing?

Click to expand...

--
http://www.flickr.com/photos/30939186@N04/

normsmith said:

It's a very overcast afternoon here today and in my house, it is dull light, I have just tried the auto focus in the house at dark locations and I am getting a focus lock at F6.3 and shutter at 1/3 without the flash strobing etc.

0.8" & F4 still works,

Click to expand...

At which ISO? Just to get an idea of light levels.

mike_2008 said:

This shouldn't be opinion based anyway, low light AF performance is a quantifiable parameter. If Carl develops a rigorous and robust testing methodology and then applies this to 4-5 cameras similar to the A33 and shows us the results then I will accept whatever result he finds... Opinions, on the other hand, are worth nothing.

Click to expand...

When Minolta would release a new camera series (700si, 800si, 9 and 7) they always advertised that their camera had the fastest and best autofocus. The photo magazine tests would confirm that Minolta cameras had fast AF and were able to use it at lower light levels (without any assist light) than any other brand. (and there was usually an asterisk with some 6 point type statement about using standardized tests.

However, in reality there were many complaints about Minolta AF, particularly when compared with Canon and Nikon cameras of the same level (and it wasn't all due to USM lenses). So obviously, there is some difference between standardized test results and actual use. Minolta was very good at meeting the specs, but something was lacking. My experience is that the Maxxum 7 had the best AF of any a-mount camera, particularly with the central cross and more so with the f/2.8 cross sensor. There was a real big difference between the speed of locking in with that sensor and any of the f/6.7 sensors.

Technical guess - Based on Minolta statements, what I've read in a couple of advance optics books on how PDAF works, and comparison with the types of sensors used by other brands (particularly in the "Canon is great" film days), I think the limits on a-mount AF is that they use f/6.7 sensors which are less accurate than the larger aperture sensors (even f/5.6).

I'll be interested in Carl's review to see how and why he came to that conclusion.

tom

The 1/3 f6.3 was at ISO 1600 - I didn't record the ISO of the second test.