<p>2 sounds reasonable. The variability you introduce by sticking to pixels or display ratios seems like it outweighs the apparent unreliability of monitor size calculations. </p>

<p>Also, unlike TVs that scale the content to fit the size, most webpages do not scale -- so, when I drag a webpage from my laptop screen to my second (larger) monitor, the size stays constant. Therefore, I would predict that people do not end up sitting as far back from large screens as you might expect. Therefore, the subtended visual angle will be approximately the same if the image is the same size for everyone.  </p>

CC BY-SA 3.0
2013-02-15T02:52:22.713Z
2013-02-15T02:52:22.713Z
Amyunimus
<p>If I managed to understand, each participant may have a slightly different viewing experience, as long as it's constant through multiple sessions each participant has.</p>

<p>To achieve this, you may ask participants to sit an arm's reach from their monitor. Try to make the images span the same physical size (even though it's really hard to achieve programatically). This way, the viewing angle would be relatively constant between participants, but more importantly, would be constant for each participant.</p>

<p>If the experiment is dependent to the level of details they can see on the pictures, you might want to consider low-res, and large-detailed images, to eliminate the advantage of those with better monitors.</p>

<p>Moreover, make sure they all use desktop computers, as many might want to user their tablet computers, or even their smartphones...</p>

CC BY-SA 3.0
2013-03-23T18:07:28.840Z
2013-03-23T18:07:28.840Z
Dvir Adler
<p>For various reasons we're doing a reaction time study involving images that runs on unstandardized equipment: patients' personal computers (unless they don't meet certain requirements).<br>
It's not about vision or individual differences, but group-level intraindividual change in reaction times (attention retraining), so it's not <em>that</em> important that participants all have the same experience as long as it's not confounded with the experimental condition.</p>

<p>Most reaction time studies run on standardised equipment, with a standardised distance from the monitor etc.</p>

<p>We can at most ask participants to keep a certain distance to the screen, but we can't ask them to buy new monitors of course.</p>

<p>So, three basic options arise (assuming our images are in a sufficiently high resolution):</p>

<ol>
<li><p>Use constant number of pixels.<br>
<strong>Positive</strong>: We can use the same images for everyone, if we use a medium number of pixels, we can accomodate most resolutions. Also, potentially everyone sees the same amount of information (i.e. no information lost due to down-scaling).<br>
<strong>Negative</strong>: With high resolutions, images may look pretty small, and with large monitors, participants may usually position themselves further away from the monitor habitually, so some see "less information".</p></li>
<li><p>Use constant display size (i.e. 7 cm wide).<br>
<strong>Positive</strong>: On large and small monitors with different resolutions, images are the same size.<br>
<strong>Negative</strong>: Apparently the information used to compute actual display size can be unreliable, so we may end up with different display sizes. We'll probably need different images and people with larger monitors and higher resolutions will be able to see more details.</p></li>
<li><p>Use constant fraction of display (i.e. 70% percent wide).<br>
<strong>Positive</strong>: Fraction of display may do the best job of letting people perceive considering that people probably sit further away from bigger monitors.<br>
<strong>Negative</strong>: Need images in different sizes (dynamically computed with JS). They may see different levels of detail.</p></li>
</ol>

<p>These options could be combined (i.e. constant pixels unless that is less than 50% of the monitor) and there's also the option of letting users zoom (will result in somewhat blurry images though).</p>

<p>Is there any research into this matter and if not, which option sounds best perception-wise?</p>

CC BY-SA 3.0
2013-01-09T10:52:52.893Z
Would it be possible for you to change the screen resolution on the patients' computers for the duration of the study? You could pick a low resolution that anyone's screen can do, and use a constant distance as well.
Ana
CC BY-SA 3.0
2013-01-09T11:18:51.003Z
@Ana We can't do it with our program (it's a web-based study). We could ask  participants to change it every time, but I think they won't like that. This would also result in different displayed sizes...
Ruben
CC BY-SA 3.0
2013-01-23T04:04:41.443Z
You can also ask on ux.stackexchange.com they may have an idea
Alex Stone
CC BY-SA 3.0
2013-03-24T13:29:23.950Z
Just a thought here, (2) would be possible, if you introduce a setup step somewhere before the test. If you create a fully scalable user interface, you could ask users initially to scale the interface to the size of e.g. a standardized dice, or just use a ruler.
Steven Jeuris
<vision><software><internet><reaction-time>
Images on personal computers - Which aspect of size to keep constant?
Ruben
# Images on personal computers - Which aspect of size to keep constant?

(Asked by: username_0 on 2013-01-08)

For various reasons we're doing a reaction time study involving images that runs on unstandardized equipment: patients' personal computers (unless they don't meet certain requirements).  
It's not about vision or individual differences, but group-level intraindividual change in reaction times (attention retraining), so it's not *that* important that participants all have the same experience as long as it's not confounded with the experimental condition.

Most reaction time studies run on standardised equipment, with a standardised distance from the monitor etc.

We can at most ask participants to keep a certain distance to the screen, but we can't ask them to buy new monitors of course.

So, three basic options arise (assuming our images are in a sufficiently high resolution):

1. Use constant number of pixels.  
   **Positive**: We can use the same images for everyone, if we use a medium number of pixels, we can accomodate most resolutions. Also, potentially everyone sees the same amount of information (i.e. no information lost due to down-scaling).  
   **Negative**: With high resolutions, images may look pretty small, and with large monitors, participants may usually position themselves further away from the monitor habitually, so some see "less information".
2. Use constant display size (i.e. 7 cm wide).  
   **Positive**: On large and small monitors with different resolutions, images are the same size.  
   **Negative**: Apparently the information used to compute actual display size can be unreliable, so we may end up with different display sizes. We'll probably need different images and people with larger monitors and higher resolutions will be able to see more details.
3. Use constant fraction of display (i.e. 70% percent wide).  
   **Positive**: Fraction of display may do the best job of letting people perceive considering that people probably sit further away from bigger monitors.  
   **Negative**: Need images in different sizes (dynamically computed with JS). They may see different levels of detail.

These options could be combined (i.e. constant pixels unless that is less than 50% of the monitor) and there's also the option of letting users zoom (will result in somewhat blurry images though).

Is there any research into this matter and if not, which option sounds best perception-wise?

**Tags:** vision, software, internet, reaction-time

> **Comments:**
> **username_1**: Would it be possible for you to change the screen resolution on the patients' computers for the duration of the study? You could pick a low resolution that anyone's screen can do, and use a constant distance as well.  
> **username_0**: @username_1 We can't do it with our program (it's a web-based study). We could ask  participants to change it every time, but I think they won't like that. This would also result in different displayed sizes...  
> **username_2**: You can also ask on ux.stackexchange.com they may have an idea  
> **username_3**: Just a thought here, (2) would be possible, if you introduce a setup step somewhere before the test. If you create a fully scalable user interface, you could ask users initially to scale the interface to the size of e.g. a standardized dice, or just use a ruler.  

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## 2 answers

### Answer by username_4 on 2013-02-15. Score: 1

2 sounds reasonable. The variability you introduce by sticking to pixels or display ratios seems like it outweighs the apparent unreliability of monitor size calculations.

Also, unlike TVs that scale the content to fit the size, most webpages do not scale -- so, when I drag a webpage from my laptop screen to my second (larger) monitor, the size stays constant. Therefore, I would predict that people do not end up sitting as far back from large screens as you might expect. Therefore, the subtended visual angle will be approximately the same if the image is the same size for everyone.


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### Answer by username_5 on 2013-03-23. Score: 1

If I managed to understand, each participant may have a slightly different viewing experience, as long as it's constant through multiple sessions each participant has.

To achieve this, you may ask participants to sit an arm's reach from their monitor. Try to make the images span the same physical size (even though it's really hard to achieve programatically). This way, the viewing angle would be relatively constant between participants, but more importantly, would be constant for each participant.

If the experiment is dependent to the level of details they can see on the pictures, you might want to consider low-res, and large-detailed images, to eliminate the advantage of those with better monitors.

Moreover, make sure they all use desktop computers, as many might want to user their tablet computers, or even their smartphones...


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