Two easy questions

S

silverJON

Suspended / Banned
Messages
340
Name
Jonathan
Edit My Images
Yes
Hello - I'm learning, but can't seem to find a definitive answer to two questions that require short answers - answers shorter than the questions, in any case!

Firstly: I understand the principle of crop factor - an old 50mm lens being like 75mm when used on my APS-C-sized sensor - but does that also apply to Pentax DA (made for digital APS-C sensor) lenses? What I mean is, does my '18-55mm' kit lens behave like an 18-55mm in 135 FOV terms when on digital (and 12-37 on actual film), or like 27-82.5 on digital and 18-55 on film? Have they already fiddled the numbers up on digital lenses to compensate?

Secondly, I don't understand at all why a digital camera needs a physical shutter - please could someone explain? I don't see why it wouldn't be far more accurate just to only accept data from the sensor for the appropriate shutter time, and leave the sensor exposed.

Thanks!

Jonathan
 
Not knowing about the Pentax DA specifically, but if it has a APS-C sensor then ~ 27-82.5 on digital and 18-55 on film.

Lots of reasons.... the most creative one being, how would allow for the effects created through a slow shutter speed? How would you stop action in a short instance of time?

The most important reason of all....... you need to control the amount of light hitting the sensor for the optimum exposure. A greater 'volume' of light needs a short amount of time on the sensor, whereas lesser amounts of light need a longer time.

No doubt through technology the need for a shutter will become less apparent but for now this is the way it is. If you have continous streaming of images how would you 'select' that instance of time you require? What happens to tall the data you don't want? How does the camera know? All image sensors (AFAIK at the moment) including the eye, suffers/has persistence of vision - a kind of memory if you like. A shutter is the best way to counteract this. If you look at a CCTV image, which is live data streaming you will wee the effects of this memory latency. The way it is at the moment is the way has been for ages at it certainly seems the best at the moment....

If you still want a couple of short answers.....

..... 27-82.5 on digital and 18-55 on film & because you do! :thinking:
 
The crop factor doesnt magnify the focal lenght of your lens. It provides you with an equiv. FOV. This means your 18-55 Pentax or Nikon or Canon for that matter will provide an equiv FOV of 27-82.5mm on a cropped sensor. The focal lenght of the lens remains 18-55mm. As DA, DX and EF-S lenses project a smaller image circle comparable to 135 format the lenses become defunct attaching them to a 135 format body, be it digital or film. The numbers havnt been fiddled they represent the actual focal lenght, not the FOV.
 
King_Boru said:
The numbers havnt been fiddled they represent the actual focal length, not the FOV.
Click to expand...

Actually, I think they have fiddled the 'numbers' to match roughly the FoV for 135 lenses..... something 'old hands' were used to....

I have two Nikkor wide angle lenses, one is a 12-24 (DX) and the other is 18-35mm (135). The DX with a 1.5 crop factor gives 18-36mm equivalent FoV.... coincidence?

An 18-55mm gives equivalent FoV ~ 36-82.5mm (35-80mm?) How about 55-200mm? Equivalent FoV ~ 82.5-300mm 70-300 (perhaps) in old money! I don't think it was by accident these focal lengths were arrived at.... :shrug:
 
The why have a mechanical shutter question got me searching around.

Answers are hard to come by. Not found anything solid enough to quote as a reference but the main factor seems to be the speed of theses large chips. They are too slow to refresh to be used for video clips and too slow for the higher shutter speeds hence the need to expose them via mechanical shutter.

There are DSLRs now that offer 'live view' on the screens but not video recording just to add to the doubt. I would be interested if someone finds a definitive answer.
 
Just a thought..... a shutter in front of the sensor helps keep the dust off it! :lol:
 
Thanks for the focal length answer everyone :) a little disappointing, because I hoped that the 28-70 I've just bought would be, like, 105mm compared to my current 55mm at the long end of my kit lens... but 70mm is still useful :)

The shutter thing: I'm well aware of the importance of shutter time - I'm in Tv mode about half the time - but I couldn't see why it had to be done with a physical blind, and not just by the processor only accepting sensor data for 1/4000 sec or however long.

Robert, is Live View done with the image sensor, or an extra little one behind the mirror?

Chuckles: I kind of assumed that the camera could control this 'streaming' and keep what it liked, but having found out that the camera does not accumulate data throughout the exposure but the sensor dumps its charge at the end, I understand a little better. I always imagined that the camera could choose a moment from the sensor's data stream like a webcam can, and save exactly the amount of data that comes in during the prescribed shutter time (electronic monostable timers are, of course, far more accurate than anything mechanical, down to nanoseconds - in theory at least).

What confuses me now is, my camera takes roughly the shutter time AGAIN to save the shot to the buffer, meaning a 10-minute bulb exposure takes 10 minutes before I can even look at it. If all it's doing is looking at the state of the sensor at the end of the exposure, why does it need so long? Surely it's the same thing whether 1/8000s or 30s?

I'm sorry that this was meant to be an 'easy question' :p
 
The 10 minutes is for noise subtraction. The camera takes an identical shot with the shutter closed and then subtracts it from the picture to remove hot pixel noise. You can probably turn it off somewhere but you will get horrible spotty pictures.
edit - it only starts working on shots over 1 second under that and it is not needed. (and thinking about it, the dark frame subtraction thing is another reason to have a mechanical shutter )

As to the live view Q - pass ;)
 
Another reason for the 'long exposure' phenomenom is the physical heat the sensor generates. This causes colour shifts due to physical hotspsots in the componentry. This has come from an observation by 'stargazers' who have been using equatorial mounts for tracking stars and long exposures. The answer was truthfully furnished by Nikon in some official publication, but I can't remember which.
 
chuckles said:
Actually, I think they have fiddled the 'numbers' to match roughly the FoV for 135 lenses..... something 'old hands' were used to....

I have two Nikkor wide angle lenses, one is a 12-24 (DX) and the other is 18-35mm (135). The DX with a 1.5 crop factor gives 18-36mm equivalent FoV.... coincidence?

An 18-55mm gives equivalent FoV ~ 36-82.5mm (35-80mm?) How about 55-200mm? Equivalent FoV ~ 82.5-300mm 70-300 (perhaps) in old money! I don't think it was by accident these focal lengths were arrived at.... :shrug:
Click to expand...

Not fiddled, just changed the focal length. The norm was a 24-70 or a 24-85 standard zoom. Now, on a cropped sensor the norm is an 18-55 or 18-70 providing you with the 27 - 82/105 FOV. Which in effect isnt as wide as the what was standard 24-70's of a FF body.

A photograph taken at 18mm with a DA, DX or EF-S lens is still 18mm. The FOV, ie, the image that is recorded, has an equiv. FOV to a 27mm lens. This isnt a magnification factor, merely a way calculationg the FOV your a seeing through the viewfinder. To magnify the image you would need a teleconverter.

You also have to remember that the DOF remains relevant to the lens focal length. So a 17-55 f/2.8, which provides a 24-80 something (had a couple of drinks) will still only provide you the subject isolation of a 55mm lens at 55mm FL. It will not provide you with an 80 odd something equiv isolation factor.

God damn,...

Bed time and time to put the Magners back in the fridge.

King.

EDIT: Crikeeey,... I'll read this tomorrow and wonder what the hell I'm on about.
 
Ok.... when I said 'fiddled' I was merely using the vernacular for 'playing around' with. I understand about crops, focal lengths, DoF and FoV - I was just trying to put into perspective how manufacturers were 'pandering' to the old lags so that they could find something they could relate to 30 years ago!

Points taken..... get a Magners out for me please! ;)
 
It also means that I, as a noob and film virgin, needn't worry my head - I never had the chance to translate focal length to FOV using 35mm (the numbers are meaningless to me) so I'm learning focal length>FOV in APS-C and will never need to convert.

...until I buy a full-frame camera, when I'm sixty-four.
 
On the shutter question I'm sure I read you shouldn't expose the ccd to bright sunshine for extended periods, I expect it is probably heat related, a bit like a magnifying glass in the sun.
 
chrisiow said:
On the shutter question I'm sure I read you shouldn't expose the ccd to bright sunshine for extended periods, I expect it is probably heat related, a bit like a magnifying glass in the sun.
Click to expand...

That sounds like another good reason. Doesn't help that lenses always sit wide open.

...although wouldn't the mirror keep the CCD nearly totally shaded anyway?
 
chuckles said:
Ok.... when I said 'fiddled' I was merely using the vernacular for 'playing around' with. I understand about crops, focal lengths, DoF and FoV - I was just trying to put into perspective how manufacturers were 'pandering' to the old lags so that they could find something they could relate to 30 years ago!

Points taken..... get a Magners out for me please! ;)
Click to expand...

:)
 
You must log in or register to reply here.
Back
Top