Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
03-22-2011, 07:20 PM   #1
Forum Member




Join Date: Jan 2011
Location: Seoul
Posts: 98
just a few technical questions about how manual lenses work

i have two questions. #1 if i am using a manual focus lens with a crop sensor, when does the crop happen? it seems to me if the sensor is not the same size as the film the lens was designed for, shouldnt what i see in the viewfinder be cropped after i take an image. if that were the case wouldnt i lose part of the image from what i saw before releasing the shutter?

#2 i am confused about how manual aperture works on a DSLR. when the lens is not mounted i can change the aperture and watch the size increase or decrease. i can see the blades moving. but when the lens is mounted and i change the aperture i feel that nothing happens inside the lens until i shot. hence the reason for stop down metering right? i wouldnt need to stop down meter if the aperture blades had moved. or if you shot in Av, even if i put the aperture ring to the smallest aperture the camera still captures the image with the aperture wide open. why doesnt manually changing the aperture on the lens do anything in Av? if the lens is fully manual how does the camera still take an image wide open???

these are not problems i am having just trying to understand the mechanics of the new camera/old lens relationship. interesting stuff. cheers!

03-22-2011, 07:33 PM   #2
Pentaxian




Join Date: May 2010
Photos: Albums
Posts: 632
This is the best place to get all your answers.
https://www.pentaxforums.com/forums/pentax-beginners-corner-q/110658-using-ma...x-dslrs-f.html

Crop factor and cropping in post processing is not same, so I understand. What you see in the viewfinder, that's what sensor records. It just means that your lens become equivalent x1.5 (taht is lose field of view but gain magnification). Hope I was clear enough.

Last edited by ultraviolet; 03-22-2011 at 07:40 PM.
03-22-2011, 07:36 PM   #3
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,360
God but I hate cropped sensor questions

The viewfinder shows between 93 and 100% of the sensor area depending on the camera, the crop as you call it is designed into the camera so you don't notice it. So forget it what you see is what you get. Just go out and shoot.

As for the aperture, when you turn the ring with the lens off camera the blades stop down, but if you press the lever that sticks out they open up. The camera is designed to hold this lever in the open position while focusing. That is called open aperture focusing. On your camera, depending on model you either press the green button or the AE button to transiently stop down the lens and set the shutter tomatch the exposure for the selected aperture this only works in manual mode. In Av mode the camera will only shoot open aperture because it does not know where the ring is set. That linkage was deleted from the standard K mount, which is why people refer to the present mount as crippled. The need for the position reading linkage was made redundant in 1981 I think, with the release of the first pentax program plus body using A series lenses. The fact that pentax carried the design forward almost 30 years after the need disappeared is impressive
03-22-2011, 07:37 PM   #4
Site Supporter
BigDave's Avatar

Join Date: Sep 2010
Location: Hudson Valley, NY
Photos: Gallery | Albums
Posts: 1,577
Hi wehav:

What you see in the viewfinder is what you will take an image of. The term "crop sensor" simply means it is smaller than the size of a 35mm frame. To understand how this would work, draw a circle. Next, put a rectangular box inside the circle such that all four corners touch part of the circle. In a simplistic sense, this is what the lens would cover if you were shooting 35mm. Now, draw a new rectangle inside the first, say about 75% of the size. This is what the cropped sensor will actually see, in simplistic terms. For the Pentax digital SLRs, there is a 1.5 multiplication factor for the "effective" focal length of the lens (the sensor is about 2/3 the size of a 35mm frame). So a 100mm telephoto on the DSLR will give the effect of a 150mm telephoto on a 35mm frame. THis is great for telephotos, not as good for wide angle. A 10mm wide angle, which would about a fisheye lens on 35mm, gives the effect of only a 15mm lens on a DSLR. Still an extreme wide angle, but not as much as a 10mm FL would be on a 35mm frame.

Regards,

03-22-2011, 07:51 PM   #5
Veteran Member
RioRico's Avatar

Join Date: Jun 2008
Location: Limbo, California
Posts: 11,264
QuoteOriginally posted by Lowell Goudge Quote
God but I hate cropped sensor questions
Ditto. I've told this story before: Back in the day, I had an Olympus Pen-FT half-frame 35mm SLR. Its frame size is about the same as a modern dSLR's APS-C sensor. I bought a Spiratone 400mm lens (cheap, of course). I thought, OH BOY, ON THE HF CAMERA IT'S LIKE A 600MM LENS! Wrong. It was a 400mm lens, with the sides of the image chopped off. That's the 'crop' -- chopping off the sides of the image. That's all. Now everyone should forget they ever heard the CRAP.FACTOR marketing term, before their heads explode. Just get out there and shoot.
03-22-2011, 08:31 PM   #6
Veteran Member
wasser's Avatar

Join Date: Jan 2009
Location: northern ca
Photos: Gallery
Posts: 427
To add to what Lowell said about aperture:

Lowell was talking with respect to K and M series lenses. There are M42 lenses (screwmount) that allow the user to switch between "manual" and "automatic" modes. In manual mode the lens will actually stop down manually as you turn the aperture ring.

The advantage of keeping the lens wide open is to allow more light in, which aids focusing.
03-22-2011, 08:38 PM   #7
Forum Member




Join Date: Jan 2011
Location: Seoul
Posts: 98
Original Poster
okay got it. thanks! sorry to push some of your buttons just by being curious about my equipment.
03-22-2011, 09:07 PM   #8
Veteran Member
wasser's Avatar

Join Date: Jan 2009
Location: northern ca
Photos: Gallery
Posts: 427
It isn't about you, it's about the confusion the whole notion of "cropped sensor" creates.

03-23-2011, 04:14 AM   #9
Veteran Member
RioRico's Avatar

Join Date: Jun 2008
Location: Limbo, California
Posts: 11,264
QuoteOriginally posted by wasser Quote
It isn't about you, it's about the confusion the whole notion of "cropped sensor" creates.
Yeah. Crap.factor and 'equivalence' are marketing terms that both sell cameras and gear, and drive numerous n00bs to the forums with the same questions, endlessly, relentlessly. It's that repetition that annoys us.

Field-of-View / Angle-of-View (FoV / AoV), as concepts, are just too nebulous to be 'sold' easily. The 'normal' lenses for any format ('normal' focal length being the diagonal of the film or sensor frame) will give roughly similar views of a subject, but all other image details will be different.

Depth-of-Field (DoF) is especially impacted by focal length equivalences. 'Normal' on a 1/1.8" P&S sensor is 9mm; on a 6x9 MF cam it's 101mm. A 9mm lens has infinitely thicker DOF than a 101mm lens at the same f-stop. This really matters. Pictures from that P&S will always look very sharp. Pictures from the MF cam won't, not without careful handling.

And that leads to the major confusion and annoyance: Folks who've moved from a P&S to a dSLR, then complain that their camera and/or kit.lens aren't sharp, the kit.lens sucks, gotta buy an upgrade now... which enriches leansmakers and dealers and puts many bargain kit.lenses on eBay, cheap. Hey, in order to approach the sharpness of a P&S, that kit.lens must be stopped-own to f/11 -- and it STILL won't be as sharp, with as-thick DOF. That's just optics.

So we see the same questions about crap.factor and equivalence and we just go ballistic. Who are the marketing wonks who devised these terms? They should be hanged. Slowly. With barbed wire. On a hot day. With ants.
__________________________________________________________________________

Oh yeah, back to the OP: To learn technical details about how lenses work, READ. At your local public library you'll likely find a fine old book, THE CAMERA, from a fine old series, THE TIME-LIFE LIBRARY OF PHOTOGRAPHY. It nicely illustrates what lenses do in various formats. Easy reading, too.

Last edited by RioRico; 03-23-2011 at 04:20 AM.
03-23-2011, 06:59 AM - 1 Like   #10
Forum Member




Join Date: Jan 2011
Location: Seoul
Posts: 98
Original Poster
how good did it feel to vent like that rico? i ONLY wanted to know if the frame i see in the viewfinder is what im getting when i take a picture. if you dont feel like explaining something you have already been through please feel free to just ignore my thread.

but on the other hand i do appreicate you taking the time to sort things out for me. so thank you!
03-23-2011, 08:36 AM   #11
Pentaxian
reeftool's Avatar

Join Date: Dec 2007
Location: Upstate New York
Photos: Gallery | Albums
Posts: 8,066
QuoteOriginally posted by wehavenowaves! Quote
how good did it feel to vent like that rico? i ONLY wanted to know if the frame i see in the viewfinder is what im getting when i take a picture. if you dont feel like explaining something you have already been through please feel free to just ignore my thread.

but on the other hand i do appreicate you taking the time to sort things out for me. so thank you!
Yes, what you see in the viewfinder on a DSLR is what your shot will be.

I can understand the confusion someone new has with lenses. Back when the majority of pictures were shot using 35mm film, the length of lenses and the field of view was the same on that auto P&S as it was on the SLR. There were smaller formats like 110 and larger like 120 but the overwhelming majority of picture taking was on 35mm. Digital sensors come in all different sizes so the camera makers use the "equiv. to xx 35MM lens" to market their cameras. So 200mm is a little more telephoto on a digital camera with an aps/c sensor which is smaller than 35mm. On a micro 4/3 camera, the telephoto effect gets even longer.
03-23-2011, 09:19 AM   #12
Inactive Account




Join Date: Aug 2008
Location: Michigan, USA
Photos: Gallery | Albums
Posts: 7,485
QuoteOriginally posted by reeftool Quote
Yes, what you see in the viewfinder on a DSLR is what your shot will be.
Actually that isn't completely accurate. What you see in the viewfinder will be on the photo but all viewfinders are not 100%. Some of the older or entry level Pentax dSLRs are something like 92%.

Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
aperture, av, blades, camera, change, crop, image, k-mount, lens, pentax lens, shot, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Manual Metering on Pentax DSLRs with Manual and Automatic Lenses Adam Pentax DSLR and Camera Articles 20 01-06-2016 03:38 AM
Manual focus questions timothy.malone Pentax DSLR Discussion 14 02-27-2011 03:47 PM
Do you use only manual focus lenses?Do a lot of folks use only manual lenses? heralu Pentax DSLR Discussion 12 01-05-2011 04:06 PM
DOF vs lens zoom (& reversed lenses) -technical discussion Yaro Photographic Technique 6 09-24-2010 08:51 AM
Can someone give me a run down of how Manual Lenses work on K-7? Abstract Pentax SLR Lens Discussion 9 09-07-2010 03:38 PM



All times are GMT -7. The time now is 01:46 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top