Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
11-15-2017, 07:29 AM - 1 Like   #1
Site Supporter
Site Supporter




Join Date: Nov 2016
Photos: Gallery
Posts: 305
Some novice questions for this month

Hello,

In the quest for more photography knowledge, I try to collect the questions that sometime I have, and put it here every now and then. Some of them may be pretty basic, but of course I ask because I don't know the answer so here it goes:

- Physically, what is really meant by "focal length"? for example the length from what to what? Why some longer lenses (focal) are shorter than some short lenses, e.g. the M 50mm f2 vs the DA 14mm f2.8? Why the 40mm can be so thin but the 35mm cannot?

- Generally, when shooting at night, say, a street, where I need a bit of DOF, assuming no tripod, is it better to go with very high ISO and low aperture, or larger aperture and lower ISO (grainy vs sharpness). I know it is subjective and dependent of the lens and camera, but what would you choose? ISO 6400 f8 or ISO 1600 f4?

- With today capability of sharpening tools such as Photoshop, Lightroom, etc, is sharpness still that important when deciding between 2 lenses?

11-15-2017, 07:41 AM - 1 Like   #2
Seeker of Knowledge
Loyal Site Supporter
aslyfox's Avatar

Join Date: Aug 2016
Location: Topeka, Kansas
Photos: Gallery | Albums
Posts: 19,912
QuoteOriginally posted by Bui Quote
Hello,

In the quest for more photography knowledge, I try to collect the questions that sometime I have, and put it here every now and then. Some of them may be pretty basic, but of course I ask because I don't know the answer so here it goes:

- Physically, what is really meant by "focal length"? for example the length from what to what? Why some longer lenses (focal) are shorter than some short lenses, e.g. the M 50mm f2 vs the DA 14mm f2.8? Why the 40mm can be so thin but the 35mm cannot?

- Generally, when shooting at night, say, a street, where I need a bit of DOF, assuming no tripod, is it better to go with very high ISO and low aperture, or larger aperture and lower ISO (grainy vs sharpness). I know it is subjective and dependent of the lens and camera, but what would you choose? ISO 6400 f8 or ISO 1600 f4?

- With today capability of sharpening tools such as Photoshop, Lightroom, etc, is sharpness still that important when deciding between 2 lenses?
1 I have copy righted all " stupid questions " but I allow others to use them without charge or penalty so ask any question you have

2 I really need to read or reread this stuff

Exposure Basics: A Complete Guide for Beginners - Articles and Tips | PentaxForums.com

A How-To for Black and White Photography - Articles and Tips | PentaxForums.com

The Fundamentals of Exposure - Introduction - In-Depth Articles

Making the Most of Long Exposure Handhelds - Introduction - In-Depth Articles

Pentax DSLR Shooting Modes Explained - Articles and Tips | PentaxForums.com

3 others will directly answer your questions I am sure
11-15-2017, 07:49 AM   #3
Bui
Site Supporter
Site Supporter




Join Date: Nov 2016
Photos: Gallery
Posts: 305
Original Poster
Thanks for your quick reply and the rich source of information, I am very interested in reading them, and will do. However, it's not that I don't understand the exposure triangle, and how to get a technically good photo, but my question is more about the physic knowledge behind them.

- For example, I know the larger the aperture, the narrower the DOF, but why?
- And similar, I understand the role of focal length in relation with field of view, DOF, bokeh, etc, but still not sure why some longer lenses are physically shorter than some shorter lenses.

:-)
11-15-2017, 07:49 AM - 2 Likes   #4
Loyal Site Supporter
Loyal Site Supporter
UncleVanya's Avatar

Join Date: Jul 2014
Photos: Gallery | Albums
Posts: 17,429
Focal length is the distance between the convergence point (where the light rays all collect at a point) and the focal plane (sensor) in whatever units used - typically millimeters in our lenses.

Different optical designs have different convergence points relative to their size length etc. Retrofocal designs impact the length and size of fast wide angles but I'm not able to clearly explain why.

Sharpness matters to a point. Sharpening doesn't eliminate the need for a clear well focused sharp image and sharpening can create artifacts.

Night shots without a tripod generally are stuck at high iso and wide apertures so the question is the very commonly an issue. Keep shutter fast enough to avoid shake induced blur and then determine what level of iso noise you can tolerate. Experiment. What I like may be different.

---------- Post added 11-15-17 at 09:55 AM ----------

Depth of field:

Explained at around 2:15s in.

11-15-2017, 08:00 AM   #5
Seeker of Knowledge
Loyal Site Supporter
aslyfox's Avatar

Join Date: Aug 2016
Location: Topeka, Kansas
Photos: Gallery | Albums
Posts: 19,912
QuoteOriginally posted by Bui Quote
Thanks for your quick reply and the rich source of information, I am very interested in reading them, and will do. However, it's not that I don't understand the exposure triangle, and how to get a technically good photo, but my question is more about the physic knowledge behind them.

- For example, I know the larger the aperture, the narrower the DOF, but why?
- And similar, I understand the role of focal length in relation with field of view, DOF, bokeh, etc, but still not sure why some longer lenses are physically shorter than some shorter lenses.

:-)
I am certainly no expert being a " seeker of knowledge " myself

but I believe that the physical design of similar lenses may be due to better technology in construction of the differing lenses with different material including metal vs. plastic, or combination there of and physical shape of internal lenses and differing material of those lenses

date of creation/manufacturing might also be a feature

if you go up to " Lenses " and check out a particular lens, the review may have a diagram of the internal lenses in that lens

da 40mm 2.8 limited

https://www.pentaxforums.com/lensreviews/hd-pentax-da-40mm-f2.8-limited.html

da 40mm 2.8 xs

https://www.pentaxforums.com/lensreviews/smc-pentax-da-40mm-f2.8-xs.html
11-15-2017, 08:38 AM   #6
Loyal Site Supporter
Loyal Site Supporter
twilhelm's Avatar

Join Date: Feb 2009
Location: Florida
Photos: Gallery | Albums
Posts: 2,355
Part of the reason a 40mm lens can be so small on a Pentax is relative to the distance between the flange and film (sensor) plane. Which is 45.46 mm so there is very little magnification needed.

(A very un-scientific explanation)
11-15-2017, 08:56 AM - 1 Like   #7
Site Supporter
Site Supporter
Alex645's Avatar

Join Date: May 2015
Location: Kaneohe, HI
Photos: Albums
Posts: 2,931
QuoteOriginally posted by Bui Quote
I know the larger the aperture, the narrower the DOF, but why?
- why some longer lenses are physically shorter than some shorter lenses.
Acceptable sharpness(or lack of) is due to points of light, called Circles of Confusion, being so small that we perceive it as sharp. With a smaller aperture, points of light that would create larger circles of light (think bokeh) cannot pass thru the small aperture, and thus create greater depth of field.

To prove this, indoors with one eye stare at your finger in front of your face. The background will be out of focus. Now with your free hand make the smallest peep hole possible with your thumb and index fingers and peep through that with one eye at your finger. You'll see now that both your finger and the background appear sharp....and that is because only small points of light could pass through your little peep hole (a.k.a. aperture).

As other have posted above, the focal length of a lens is the distance between the nodal point (where the light criss-crosses inside the lens) and the film plane or sensor. Longer focal length lenses that are physically shorter from the front to rear element is because of the how the optics were designed internally. This may compromise some optical qualities which is why the higher end Zeiss or Sigma Art lenses tend to be larger, but not always.

Last edited by Alex645; 11-15-2017 at 03:21 PM.
11-15-2017, 09:25 AM   #8
Pentaxian




Join Date: Nov 2015
Location: Bremen
Posts: 358
There is a simple formula that answers your sharpness question: garbage in equals garbage out. Meaning that if the image is not reasonably sharp in the first place you won’t be able to make it stunningly sharp. Of course acceptable sharpness always depends on publishing size. The sharpness of a picture might be enough to post on instagram but not enough to make a huge print.

Also this might change a bit in the future with sharpening algorithms getting better and algorithms that are being developed that can even recover blurred images (have a look at http://yuzhikov.com if you’re interested to see what’s possible right now)

So to answer your question, it really depends on what you want to do with your images. If you only post them on instagram the kit lens will probably be fine. If you plan to show them on huge screens or make big prints then you have to use high quality glass to get the best results.


Last edited by alpheios; 11-15-2017 at 09:58 AM.
11-15-2017, 10:02 AM - 1 Like   #9
Site Supporter
Site Supporter




Join Date: May 2016
Photos: Albums
Posts: 1,544
The simple version of "focal length" is that it is the distance of the lens to the focal plane (the camera sensor in this case). But it refers to a very simple lens -- one element -- and in this case the focal length is very clear. With modern camera lenses, there are multiple glass lens elements in a lens that you mount on your camera. and the focal length marked on the lens may not correspond to the actual distance from a glass element to the sensor. In these cases, "focal length" corresponds to the focal length of the equivalent simple lens that gives you the same field of view as the camera lens.

One of the other links will probably explain it better, but the Wikipedia page shows the "simple lens" example:

Focal length - Wikipedia
11-15-2017, 10:07 AM - 1 Like   #10
Pentaxian




Join Date: Jan 2011
Location: New York
Posts: 4,643
Here's my attempt at answering.

Q1: Physically, what is really meant by "focal length"? for example the length from what to what? Why some longer lenses (focal) are shorter than some short lenses, e.g. the M 50mm f2 vs the DA 14mm f2.8? Why the 40mm can be so thin but the 35mm cannot?

A1: In practical terms, focal length indicates how much a lens magnifies, and what sort of angle of view you'll have. For a theoretical single-element lens, focal length equals the distance from the glass to the focal point, so a 40mm lens would be exactly 40mm in front of the sensor. In practice, camera lenses have multiple pieces of glass to control chromatic aberration, get a flat focal plane, etc. and the length of the lens varies from that theoretical length. Achieving a compact lens size requires trading off performance in other areas.

Q2: Generally, when shooting at night, say, a street, where I need a bit of DOF, assuming no tripod, is it better to go with very high ISO and low aperture, or larger aperture and lower ISO (grainy vs sharpness). I know it is subjective and dependent of the lens and camera, but what would you choose? ISO 6400 f8 or ISO 1600 f4?

A2: The only night situation that I would shoot without a tripod is street photography. I might use ISO 6400 f8 if I wanted foreground and background to both be sharp (and accept the extra noise), or ISO 1600 f4 if I wanted to isolate the foreground and let the background be blurry.

Q3: With today capability of sharpening tools such as Photoshop, Lightroom, etc, is sharpness still that important when deciding between 2 lenses?

A3: Yes. It depends on your photographic style and how large you want to display/print your photos. A very sharp lens also allows you to crop the photo to magnify a small part of the scene. It is difficult to sharpen or crop a blurry photo. IMO sharpness is most important for photos of the night sky (stars should be in focus across the frame which requires a flat field instead of just center sharpness), small birds (cropping likely to be needed), and macro.

Once a lens is sharp enough, and that's subjective with each photographer, other lens qualities become more important. The Pentax DA 15 Limited is a good example. It's arguably Pentax' most distinctive lens. It's sharp enough for me but nit tack-sharp if you pixel peep. What do I get in exchange for the lack of perfect sharpness?: legendary flare resistance, contrast, and a very compact size.
11-15-2017, 05:23 PM   #11
Pentaxian




Join Date: Dec 2016
Location: Southeastern Michigan
Posts: 2,774
A simple way to increase your DOF with night shots is to use longer exposures so you don't have to rely on higher ISO in order to use smaller apertures, and thus compromise quality. Also, go for normal to wide angle views to avoid using tele lenses, since the longer focal length will produce less DOF within the frame, requiring yet smaller apertures. Or, if your shot is of a tele nature, make your field of interest all at a considerable distance.

The downside of using longer exposures is they may need to be slow enough to be beyond the capability of your SR technology, so you must use a tripod. Any movement within the composition will also likely appear as a blur.

Juggling the above factors are good reasons why photographers get excited over new camera equipment having improved higher ISO performance, including lower noise and better image quality retention, better AF in low light, and improved SR performance for hand-held shots.

Last edited by mikesbike; 11-15-2017 at 08:36 PM.
11-16-2017, 12:41 AM   #12
Loyal Site Supporter
Loyal Site Supporter
pschlute's Avatar

Join Date: Mar 2007
Location: Surrey, UK
Photos: Gallery
Posts: 4,439
QuoteOriginally posted by mikesbike Quote
Also, go for normal to wide angle views to avoid using tele lenses, since the longer focal length will produce less DOF within the frame
In the interset of clarity for the OP, the focal length of a lens in itself does not affect DOF. If the same picture is taken with a 50mm and a 100mm lens so that the subject is the same size in the image , both pictures will display near identical DOF.for any given aperture. (The camera to subject distance will be twice as long with the 100mm lens).

Of course if the camera to subject distance remains constant, the DOF will be narrower with the 100mm lens, but this is a result of the magnification, ie. the subject will be twice the size in the picture.
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, depth, designs, field, iso, k-mount, length, lenses, novice questions, pentax lens, questions, sharpness, size, slr lens, vs
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
The Grass Is Greener? Some Thoughts Over Shooting With Nikon For The Last Month reivax Pentax DSLR Discussion 17 10-10-2016 01:14 PM
Questions, questions, questions (PZ-1 and P3) dakight Pentax Film SLR Discussion 12 09-06-2014 06:23 AM
General questions from a novice. Finknottle Troubleshooting and Beginner Help 7 01-28-2012 04:31 AM
Novice Here (Cheap Flash Questions) Globaldanish86 Flashes, Lighting, and Studio 9 06-21-2010 02:48 AM
Some scenes from the last month dugrant153 Post Your Photos! 3 02-02-2009 05:17 PM



All times are GMT -7. The time now is 02:01 AM. | 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